Bristol Jupiter VIII F., VIII F.P., XI F., XI F.P.
Part 1: Specifications and Ratings, Description
Compiled by Kimble D. McCutcheon
Published 26 Mar 2025; Revised 1 Apr 2025

Bristol Jupiter VIII F. (Kogo)

The air-cooled 9-cylinder ristol Jupiter VIII F., VIII F.P., XI F. and XI F.P. radials, which appeared in 1929, were drastically different than the previously-covered Jupiter IV. Although the 5.75" bore, 7.50" stroke and 1,752.79 in³ displacement was retained, nearly every part was newly designed and propeller reduction gearing running at 1/2 crankshaft speed was added. The VIII F.introduced forged cylinder heads.The VIII F.P. featured pressure-fed wrist (knuckle) pins for improved durability. The Jupiter XI F. and XI F.P. had lower compression ratios and ratings, but were otherwise similar. They all had composite cylinders with forged Y-alloy heads and steel barrels that were screwed and shrunk into the heads. Many thanks to Bruce Vander Mark for providing the manual from which most of this article was taken.

Bristol Jupiter VIII F.

Specifications and Ratings

Introduction

Minor changes included the introduction of ball-ended quickly-detachable high-tension sparking plug terminals and a specially-offset gas starter union for cylinder No. 5 to clear the offset tie rods necessary on this cylinder because of the oil sump.

The F. and F.P. types differed in their wrist pin oil feeds. F. type engines used a semi-pressure system where surplus oil from the master rod big end bearing was caught in a collector ring bolted on to the rod front end where centrifugal force caused it to flow in ducts leading to the hollow wrist pin bores. Since there was an appreciable working clearance between the rod and crank, this arrangement was not a pressure system. In F.P. engines the collector ring capped the rod bearing ends in a close sliding fit. Three springs captured between the rod and the ring pressed the ring axially into working contact with the crankpin front end. A thrust ring that was a tight fit over the rod rear end completed the seal. Oil leaving the big end and collected in the collector ring was therefore fed under pressure to the wrist pins.

Description

Crankcase

Crankcase

Front Cover, Front Half, Rear Half	Rear Cover, Spiral, Induction Chamber Cover	Main Bearing Mounting	Engine Mount Attachment

Four major assemblies comprised the crankcase: the front cover, crank chamber,  rear cover and induction chamber cover. A small detachable oil sump was fitted at the crank chamber's lowest point and received oil draining from the engine. The nine-sided crankcase accommodated nine cylinders equally spaced around it. The crank chamber was built up from two duralumin drop forgings, the front and rear halves of which were machined to shape and met in a faced joint on the cylinder center lines. The two halves were secured by nine close-tolerance bolts that served to register the joint. The bolt holes were bored in the nine angles formed between the cylinders. The bolts were made with collars about 2.25" from their rear ends and these were sunk in recesses provided for them in the rear half rear face. The bolts were prevented from turning by small dowels on the collars that fitted into notches cut in the recesses. The guides for the two bolts on either side of the uppermost cylinder were transversely slotted to permit two engine lifting eyes to enter the guides and be threaded onto the bolts. The nine crankcase sides surrounding the crank chamber were faced and cut with apertures to receive the nine cylinders. Each cylinder was secured by eight waisted studs, four in each half of the crankcase, which were located in pairs at the cylinder base flange corners. The studs were screwed into the crankcase and were further secured by nuts on the inner ends; these nuts were locked by peening over the studs. The cylinder apertures were made with a concave chamfer at their edges that accommodated a rubber ring on the cylinder spigot to prevent oil leakage.

The rear main crankshaft roller bearing was housed in the crank chamber rear wall and the intermediate main roller bearing in the front wall. The bearings' outer rings were shrunk into aluminium-bronze housings, which were then shrink-fitted into the crankcase wall. The aluminium bronze housing in the crankcase wall was keyed by three taper pins driven into corresponding holes cut through the bronze and into the crankcase wall from within the housing. The holes were carried through to enable the pins to be knocked out should housing replacement be necessary. No provision was made for securing the outer ring bearing against rotation in the aluminium-bronze housing, but a spring steel ring that snapped into a groove in the bronze provided axial bearing retention

An annular induction chamber at the crankcase rear was formed partly by the crankcase rear cover rearward extension and partly by the induction chamber cover. Around the chamber's crankcase portion were nine apertures faced to receive induction pipe flanges that radiated to the cylinders and were secured by four studs each. Three ports in the cover's lower segment were faced to receive the carburettor assembly, which was attached by four studs and nuts at each aperture. The cover was secured by two rings of studs around its inner and outer circumferences, the joint being sealed with special paper washers and jointing compound. Within the casing was housed a circular three-start spiral that distributed the mixture from each carburettor barrel to the three cylinders it served.

A faced flange formed around the inner circumference of the induction chamber crankcase portion received the crankcase rear cover, which spigoted into the flange and was secured by ten studs projecting from the flange. The studs were screwed into the flange and were further secured by split-pinned locknuts. The nuts securing the rear cover were secured with locking plates. The rear cover formed the base for two magnetos, the oil pumps and filters, the hand turning gear, the fuel pump drive and the revolution indicator drive. The white-metal bearing for the sleeve at the crankshaft rear end was also housed in the rear cover center.

The oil pump was in a housing immediately behind the crankshaft, which drove it directly. Oil passages to and from the pump were made in the rear cover body and three oil pipe unions were mounted on the rear face. Two deep filter chambers were cast on either side of, but below, the oil pump's axis; these housed the pressure and scavenger filters. An external oil sump was attached at the crank chamber's lowest point, between cylinders 5 and 6; it received oil draining from the engine interior. The two hollow limbs that projected from the sump top provided the means for its crankcase attachment and also conducted oil into it; faced flanges made at the limb upper ends connected with appropriate apertures at the crankcase bottom using paper washers and liquid jointing compound. The metal was cut away on each side of the lowest crankcase bolt to receive the attachment. The faced crankcase surfaces to which the sump attached included passages by which the oil drained from the crankcase and the front cover. Three holes in the faced surfaces at the crankcase front included two that communicated with the crank chamber interior and one with the front cover, while two holes in the rear faced surface communicated with the crank chamber interior only. The metal around the holes and between the adjacent cylinder studs was grooved to facilitate oil flow to the holes. Two passages were made in the rear wall to drain the space enclosed by the rear cover; these broke through the rear wall front face immediately above the two drain holes to the sump and were shielded by a small bolted-on aluminium louver to prevent flow interference by the revolving crank.

Three large holes in the crank chamber upper segments between the front and rear walls connected the rear cover, crank chamber and front cover to equalize pressure throughout the engine.

The magnetos were mounted on two rear-projecting bosses on either side of the oil pump and whose axes converged at 32° from the crankshaft center line. The bosses housed the magneto drive couplings and magneto drive shaft bearings. The hand turning gear and the fuel pump drive were mounted in line around a common layshaft that transmitted their drives and was geared directly to the crankshaft.

A circular spigot around the crankcase outer periphery attachrf the engine to the airframe bearer plate; the nine crankcase bolt stub ends projected through the spigot face and thereby attached the engine to the plate. Two concentric rings projected from the crankcase front face and together provided for the nine pairs of tappets and guides that were positioned radially around the crankcase. The outer ring was integral with the crankcase and was strengthened by nine webs formed between it and the crankcase. The cam gear was housed within the inner ring, which was spigot jointed and bolted to the crankcase wall. Nine pairs of faced bosses around the outer ring received the tappet guide flanges. Four studs at each facing carried the tappet guide flanges, the tappet oil guard plates and a pair of cages to house the push rod springs. The guard plates covered the tappet guide tops except for holes only sufficiently large to admit the push rods. This scheme masked the large tappet guide open end area, helping to prevent grit or foreign matter from entering the tappet cup.

The front cover, a forward-dished aluminium plate, closed the crankcase front. It was formed with a deep forward-extending rim around its outer edge, which was made with a spigot at the front to receive the reduction gear casing. The front cover was attached to the crankcase by 18 studs and split-pinned nuts. The fixed internal gear, a cam gear component, was anchored between the cover and the crankcase. The internal gear was made with a spigot on both its front and rear face to register with the crankcase and the front cover respectively. The front main crankshaft bearing was centrally mounted in the front cover wall. It was contained in a flanged steel housing bolted to the wall by eight studs and split-pinned nuts. The bearing's outer ring was retained between a lip in the housing and a spigot formed on the front cover on to which the housing was mounted.  Eight internal radial stiffening ribs and the gas starter distributor drive housing were formed between the bearing aperture and the rim. The bays between the ribs, with the exception of the three lowest ones, were perforated with large holes that maintained equal pressure between the reduction gear casing and the engine interior. The lowest bays were not perforated, as they formed a partition to maintain the requisite oil level in the reduction gear casing. The gas starter distributor was mounted to a surfaced face at the front cover top using four studs and spring-washered nuts. The long downwardly extending distributor casing limb was formed with a flanged spigot at the upper end that spigoted into the mounting and located the casing, while the lower end was located by another and smaller spigot that entered a suitably-reamed aperture. The aperture was made in the roof of the small box-like housing, cast integrally with the front cover, that accommodated the small drive shaft for the gas distributor. Two small brass breathers with internal gauze cones were fitted to two boxed apertures on either side of the gas starter distributor.

Crankshaft

Crankshaft and Assemblies

The single-throw two-piece crankshaft was machined from chrome nickel steel forgings joined at the crankpin rear end. The crankwebs extended across the shaft center line to the opposite side where they were fitted with counterweights to balance the crank, the connecting rods and the pistons. Tenons extending across the weights entered corresponding slots in the webs to locate the weights. Four countersunk setscrews with inset nuts were used to secure the weights to the web; the screws were peaned over to lock the nuts in position. Lead fillings inserted in holes sunk in the rear web extension provided compensation for the reciprocating parts, but as these fillings were covered by the counterweight they were not normally visible. The counterweight web extensions were made with lightening pockets on their inner faces. The joint between the two crankshaft parts was of simple construction. The rear crankweb was bored and split to receive the crankpin rear end, which was slightly stepped at the end to enter the hole; the crankpin was gripped in the hole by a massive transverse clamp bolt that passed through the top of the split web and was drawn up by a split-pinned nut. A key integral with the web and projecting into the hole on the side opposite the split entered a corresponding keyway in the crankpin, thereby locating the two pieces in respect of each other. The key was a close fit in the keyway to ensure true alignment of the crankwebs and to prevent movement between the two shaft portions. The crankshaft was borne in one ball bearing and two roller bearings, which were known as the front, intermediate and rear main crankshaft bearings. The rear bearing adjoined the rear crankweb rear face and the intermediate bearing the front crankweb front face. Both bearings' inner rings of were chamfered to clear the radius formed at the shaft/ web junction. These two bearings took the main journal load and were of the crowded loose roller type, the rollers being located in a groove in the inner ring and working in a plain ungrooved outer ring. This arrangement permitted the shaft to be centralized in the crank chamber and also permitted bearing alignment when the crankcase expanded slightly under the heat of running conditions. The front main bearing was a caged ball bearing and served chiefly to steady the crankshaft front end. The rear main bearing inner ring was secured in position by a conical sectioned distance piece that received pressure from a ring nut securing the hand turning gear spur wheel that was mounted on the crankshaft rear. The intermediate bearing inner ring was drawn up against the front crankweb by the assembly of fixed items on the crankshaft front (described later), all secured by a left-hand-threaded front main bearing nut that drew up the front main bearing inner ring and all the fixed items on the shaft front end. The nut was secured by a locking washer with a tab that engaged a slot in the shaft. A splined taper machined immediately in front of the bearing-holding thread was also threaded to accommodate the reduction gear driving member and its securing nut. The crankshaft rear end was of much lighter construction than the remainder, as this portion served only to drive the auxiliaries.

Oil Plug and Baffles

The crankpin and the crankshaft main journals were hollow; passages drilled in the webs to completed the oil paths. Each end of the hollow crankpin was plugged by a hexagon-headed aluminium screw plug, which was secured in position by a locking washer. The crankshaft rear end was open and received the oil delivered into it by the pump, the driving member of which was coaxial with the shaft and was driven by it. The front end of the rear portion was closed by an threaded aluminium plug locked by a large tab washer, the top of which was fixed into a hole sunk for it in the crankweb, while the washer itself was bent up to engage a flat on the plug. The crankshaft front section bore was enlarged at its rear end and threaded, the enlarged diameter joining the bore in a steep taper. An aluminum flush-fitting combined plug and oil baffle was screwed in and served the dual role of plugging the bore rear end and of restricting the quantity and pressure of the oil entering the shaft front portion, which was required solely to lubricate the reduction gear. The plug extended forward in the form of a steep cone, considerably steeper than the conical recess in the shaft, thus providing an annular oil space between the two into which the oil passage in the crankweb opened. At the end of the cone was a baffle that consisted of a parallel-sided plug that fitted the shaft bore and upon the outer surface of which nine cavities were machined. When the plug was in position in the bore, these formed nine small chambers that were interconnected by a series of holes through which the oil was forced. The effect of forcing the oil through these holes was to reduce its pressure and flow by successive stages until the quantity was proportionate to the requirements of the reduction gear. The combined plug and baffle was locked in position with a tab washer secured to the face of the crankweb by a small setscrew.

A white-metalled phosphor bronze bush pressed into the crankshaft bore front end constituted the airscrew shaft rear bearing; the bore was slightly increased in diameter here to accommodate it. Oil grooves were provided in the bush bore. Two slots were cut in the rear end to receive an extractor for removing the bush for replacement purposes. Forward of the bush the crankshaft was bell-mouthed to receive the airscrew shaft spigot. Three shallow longitudinal oilways machined for a short distance on the crankshaft front portion immediately in front of the crankweb supplied oil to the mechanisms at the shaft front. A duct drilled in the web extension in line with one of the oilways communicated with the main oil passage within the web. The crankpin was drilled with two ducts to supply the big end bearing and a flat was made on the pin where these occurred to distribute the oil. The end of the oil passage down the front web was closed by a threaded plug locked with a tab washer.

Rear Cover with Auxiliary Drives

All the auxiliary drives except the gas starter distributor were taken from the crankshaft rear. Two keyways cut on the shaft rear end engaged the magneto driving bevel sleeve, which was attached to the magneto shaft end and ran in a white-metal bearing in the crankcase rear cover. The bevel was retained in its bearing by a split phosphor bronze keep that secured a collar made on the sleeve immediately behind the bevel wheel; the keep was attached by four studs on the rear cover front. The keep also served to clamp the white-metal bearing in its housing. The bearing was secured against rotation by four hardened steel washers threaded onto the studs, which were received in scallops made in the bearing flange. The washers were the same thickness as the flange and provided a true surface for the keep to bed upon. The bearing flange was originally made with four drilled lugs for the studs, but the pressure of the retaining nuts buckled the keep plate and crushed the lugs. The steel washers were employed to provide the necessary support.

The oil pump was driven directly from the crankshaft rear end, its coupling consisting of a drilled flange formed on the end of the pump driving spindle. Two slots in the flange engaged the ends of the two keys by which the magneto driving bevel was keyed to the shaft. The shaft rear end opened into the chamber in which the oil pump was housed, and oil from the pump flooded the chamber and therefore the shaft, providing a simple means of feeding the shaft with oil.

The hand turning gear drive and fuel pump drive were taken from a layshaft mounted in the rear cover and driven by a spur wheel mounted on the crankshaft rear end. The spur wheel, which was of substantial proportions, had to transmit the hand turning gear load. It was mounted on a parallel portion of the shaft and was keyed by six splines formed on the shaft, which engaged with corresponding slots in the wheel. The spur wheel was secured by a ring nut on a threaded portion of the shaft immediately behind; the nut was locked in position with the tab washer. The spur wheel abutted a conical distance piece threaded on to the shaft in front of it, and this in turn bore on the rear main crankshaft bearing inner ring and secured it in position.

The crankshaft front end carried the cam gear between the intermediate and the front main crankshaft bearings. A spur wheel mounted on the cam plate provided a drive for the gas starter distributor, which was carried on the front cover. The cam gear was mounted upon a sleeve that was slid onto the shaft and this sleeve, together with other fittings on the shaft, served as distance pieces that were drawn up by the front bearing nut.

Connecting Rods

Connecting Rods

Articulated, Master	F.P. Type Wrist Pin Lubrication	F. Type Wrist Pin Lubrication

The master connecting rod had eight articulated rods grouped around its big end. The rods were of high tensile-strength steel. The articulated rods were coupled to the master rod big end by wrist pins that were mounted in a pair of flanges formed around the big end; the articulated rods were captured between the flanges. The master rod big end was fitted with a one-piece floating steel bush that was white-metalled inside and outside to provide a suitable bearing surface for the crankpin and the big end bore. The bush was drilled to ensure effective lubrication, the holes being chamfered on both surfaces. The bush projected beyond the master rod big end at each end. The projection accommodated the thickness of a phosphor bronze oil retaining ring at the front and a phosphor bronze thrust ring at the rear. These components formed part of a lubrication system for the wrist pins (discussed later). The H-section master rod shank gradually decreased in cross-sectional area from the big to the small end. A floating phosphor bronze gudgeon pin bush was fitted (except in some later F type engines in which the bushes were fixed in the rod), and holes were drilled in both the bush and the rod to provide gudgeon pin lubrication. The pair of flanges around the master rod big end was machined with eight holes for the articulated rod wrist pins. The wrist pin holes were pitched at slightly varying radii from the master rod big end center to help compensate for the unequal articulated rod strokes. The hole center radius pitching was arranged in pairs so that the center radius of any one wrist pin was equal to that of its fellow on the opposite side of the master rod center line. A small arrow engraved on the big end front face indicated the direction of rotation and guarded against incorrect rod assembly on the crankpin.

The hollow wrist pins included an internal collar that served to anchor the pin using a short securing bolt. A short taper was ground on one end, followed by a length of uniform diameter upon which the articulated rod ran, and then a slightly larger diameter section that fitted into the aft master rod flange. Suitably-tapered holes in the master rod forward flange received the tapered wrist pin ends. A steel cup, larger in diameter than the pin, closed the tapered end and anchored the pin against the master rod forward flange. The securing bolt was inserted from the wrist pin's opposite endand projected through the cup where it was secured by a split-pinned nut. Wrist pins were lubricated by oil forced out from the master rod big end bearing. The big end was flanked by two oil retainers in the form of flanged rings, of which the front one was a close sliding fit and the rear one a tight fit on master rod big end. The floating bush's ends projected into the rings. The rearmost thrust ring received the end thrust imposed by three equally-spaced small helical springs captured between the foremost oil retainer ring and the master rod. The springs were housed in three drillings in lugs projecting from oil retainer circumference. One of the lugs extended to fit between the washers on Nos. 5 and 6 wrist pins to key the ring and prevent it from rotating on the rod. Oil passing out the master rod front end was collected in a small annular space formed by a chamfer cut on the master rod front end and a corresponding chamfer in the oil retainer. Eight oil passages drilled in the master rod from the chamfered front end to the tapered wrist pin seats were met by holes drilled in the hollow wrist pins leading to the chamber formed by the retaining cup. From there oil flowed to two flats on the wrist pin surface via two oblique holes in the pin. This oil flow was assisted by four scallops cut in the master rod bearing bush front end. Another chamfer around the thrust ring rear face inner circumference helped lubricate this face.

In Jupiter F. type engines the oil to the wrist pins was semi-pressure fed. An aluminium oil-catcher ring was fitted to the master rod front face where it was secured by the nuts on the wrist pin bolts, replacing the individual washers. An internal groove within the ring matched up with the radius on the master rod big end front to form an annular chamber in which the oil was caught. Eight inclined passages leading out of the chamber delivered the oil to the wrist pin front bore where it was led, by two inclined passages in each pin, to flats on the wrist pin's working surface.

The articulated rods were of uniform section throughout their length and both ends were bushed with floating phosphor bronze bushes (except in some later F type engines in which the bushes were fixed in the rod). Lubrication holes were drilled in both the rod and the bush at the gudgeon pin end. With floating wrist pin bushes, three scallops were cut in each end of the bush and also in each end of the rod eye.

Pistons

Numerous piston designs were used in Jupiter VIII and XI engines; and are classified in Table 1.1 and illustrated in Figure 11.

Piston Details

Fig. 11. Types	Sectioned	Rings, Gudgeon Pin	Gudgeon Pin

The early pistons (Table 1.1, #s (1), (2), (5), (8)) comprised a series of cast Y-alloy pistons of which (1) and (2) were at first fitted in all nine cylinders. Later it was found necessary to fit a special piston in cylinder No. 6 to resolve the additional master rod loads, and (3) was fitted in cylinder No. 6 while (1) and (2) were replaced by (5) and (8) for the remaining eight cylinders. Piston (1) had a full skirt on the leading side and a deep scallop on the trailing side, while in (2) scallops were cut on both the leading and trailing sides. When it was found necessary to fit a special piston in cylinder No. 6, (3) was scalloped on the trailing side only, thereby maintaining the maximum working surface on the leading side where it was needed to meet the load imposed by the master rod on that surface. A curved oil groove was also introduced on the leading face to ensure more efficient lubrication there. This piston was superseded by (4) and (6), which were of similar design but provided closer working clearances in the cylinder. Pistons (4) and (6) were in turn replaced by (7) in which the clearances were even closer. In these three pistons the drain holes (previously drilled in the oil drain groove below the scraper ring), on the leading and trailing piston sides were omitted to conserve the oil at these points. Piston (9), which was fitted in cylinder No. 6 only, differed from the remaining pistons in that its leading side was ground to a larger radius than its trailing side to correspond more closely to the radius of the cylinder bore in order to effect more satisfactory lubrication and to increase the bearing surface on the leading side. The trailing side was scalloped and the leading side was grooved for oil, but instead of the single curved groove two straight grooves were cut diagonally across the face. The oil groove below the scraper ring was interrupted at the leading and trailing faces to conserve the oil at these points.

The first forged piston was (10), which was fitted to cylinder No. 6 only and was superseded by (11) in which the main difference was that the oil holes in the gudgeon pin bosses were staggered. Piston (11) was replaced by (12) in which the gudgeon pin bosses were strengthened, making (12) the standard piston for cylinder No. 6. Following the introduction of forged cylinder No. 6 pistons, forged pistons were introduced for the remaining eight cylinders, and the development of these followed the lines of the piston for cylinder No. 6. The first forged piston for the remaining eight cylinders was (13), followed by (14) in which the oil holes in the gudgeon pin bosses were staggered. Piston (14) was superseded by a piston with strengthened gudgeon pin bosses (15), which became the standard piston for all cylinders except No. 6.

With the introduction of forged pistons the material specification was changed from Y-alloy to D.T.D. 132, and the piston design was changed from the slipper to the full skirt type. The curved oil groove on the leading face for cylinder No. 6 pistons was also deleted, and the oil collector groove, which in the cast pistons (except (9) extended around the full circumference, was in these pistons, as in the case of (9), interrupted at the leading and trailing faces. Pistons for the remaining eight cylinders, (13), (14) and (15), still had a groove cut around the whole circumference and was drilled on both the leading and trailing faces to conduct oil back to the crank chamber. The pistons in cylinder No. 6, (10), (11) and (12), were scalloped on the trailing edge only, while those in the remaining eight cylinders, (13), (14) and (15), were scalloped on both the leading and trailing edges. All full skirt pistons were slightly scalloped under the gudgeon pin bosses, which enabled the full skirt to clear the crankshaft balance weights when the piston was at the bottom of its stroke. All pistons had concave crowns and all were fitted with three cast iron rings. The lowest was an oil scraper and was wider than the two compression rings. It was grooved midway to a U-section and holes were drilled through the groove bottom to conduct oil to drain holes in the piston at the back of the ring groove.

A hollow fully-floating gudgeon pin of air hardened steel was a working fit in the gudgeon pin bosses and was retained at each end by an air hardened steel washer. The washer fitted over the pin's end and was secured by a piano wire circlip that snapped into a groove on the pin. One side of the washer was ground to an inwardly-inclined chamfer to assist in retaining the circlip. The washer had a square face on the opposite side, and was of such a thickness that it was impossible to reverse it, or fit it in an incorrect, position. Each circlip could be used only once, as the action of removal destroyed its grip. An ample pin end clearance of 0.040 – 0.060" allowed for piston expansion. Gudgeon pins in the forged pistons were longer than those in the cast pistons, and were not interchangeable, although the circlips and washers could be used for either. Gudgeon pin lubrication was via holes in each gudgeon pin boss and holes in the small end and small end bush. In cast pistons a short lugwithin the piston on either side between the two webs that carried the gudgeon pin bosses prevented the skirt from falling against the connecting rod when the cylinder was removed. Therod encountered the lug before reaching the skirt. These lugs were omitted in forged pistons .

Cylinders

Cylinder

The composite cylinders consisted of a steel barrel and a forged Y-alloy head that was permanently screwed and shrunk onto the barrel top. Each cylinder was fitted with two inlet and two exhaust valves, which were operated by overhead rockers mounted on the cylinder head. The two exhaust ports were at the cylinder front, and in plan view, situated at 31° to the engine center line. The two inlet ports were at the rear and faced directly aft.

The barrel was machined from a steel forging and was provided with cooling fins of equal diameter except for the six lower ones, whose diameter decreased towards the bottom. Finning did not extend down to the base flange; a belt about an inch wide was left between the lowest fin and the flange. The base flange was approximately square in shape with each corner machined to form a pair of lugs, each with a hole drilled to receive a holding-down stud, thus making eight studs for each cylinder. A circular rib was formed around thebase flange top to strengthen the flange and minimize distortion.

Cylinder construction was the primary distinguishing feature between the Jupiter VIII F. and VIII F.P. series and the Jupiter XI F. and XI F.P. series; the barrel length determined the compression ratio. The VIII F. and VIII F.P. engines with the 5.8:1 ratio were fitted with a 17-fin barrel, while the XI F. and the XI F.P. engines with a 5.15:1 ratio had an 18-fin barrel, along with a 0.065" thick mild steel packing washer fitted between the base flange and the crankcase. The cylinder spigot extended into the crankcase to a depth of about 2.2". The master rod was fitted in cylinder No. 6. Cylinders Nos. 5, 6 and 7 spigots were scalloped at the trailing edge to provide connecting rod clearance, while the remaining cylinder trailing edges was cut with a deeper scallop and the leading edge with a rectangular clearance area for this purpose. Because of the master rod/articulated rod geometry, the clearance area required for the cylinder No. 6 spigot and the adjacent cylinders was less than that required for the remainder. The master rod cylinder was therefore located in the lower segment of the engine, as by this arrangement the cylinders with the shallower spigot clearance areas were at the bottom and tended to prevent the bottom cylinders from becoming over-oiled. The joint between the cylinder and the crankcase was sealed by an oil-tight rubber packing ring fitted on to the cylinder spigot and accommodated in a concave-sectioned chamfer made around the crankcase aperture. Mineral oil was poured on to the rubber ring when the cylinder was being assembled; this caused the rubber to swell after the holding-down nuts were tightened, and made an oil-tight joint.

The external barrel surface above the top fin was threaded to receive a corresponding cylinder head thread. The joint between the head and barrel was made by heating the head to 320°C and screwing it on to the cold barrel. As the diameter of head was 0.020" smaller than that of the barrel at the joint, a firm shrink fit was obtained. The solid copper jointing washer prevented hot gases from contacting the joint threads. The copper washer was accommodated in the square chamfer around the barrel top on the one part and in a recess in the other. The head was machined parallel below its lowest fin to accommodate a solid steel ring known as the grip band, which was shrunk in position after the joint has been made. By the differential expansion provided by the steel ring and the alloy head, a compressive stress was exerted on the lower part of the joint. Once manufactured, the joint and grip band were never disturbed.

The Y-alloy cylinder head was heavily finned all over, both horizontally and vertically, to ensure efficient cooling. The combustion chamber roof was of penthouse form, the included angle between the two flat surfaces, which extended from front to rear, was 120°. Two apertures were cut in each surface and were fitted with aluminium nickel bronze valve seats, which were screwed, shrunk and peened in position. The seats were cut with a 45° chamfer. The apertures led to gas passages formed in the head. The two front-facing exhaust passages terminated in two faces with four studs each for attaching the exhaust branch pipes; the inlet passages at the rear had two similar faces with two studs for the inlet branch pipes. The valve axes were perpendicular to the combustion chamber roof surfaces resulting in a 60° included angle between the valve stems and cylinder axis. The valve guides were housed in parallel bores in cylinder head bosses that protruded into the gas passages. The upper end of each bore developed into a conical section to accommodate a corresponding shoulder towards the valve guide top. The shoulder upper face was flat to receive the valve spring washer.

Two sparking plugs were mounted one on each side of the cylinder head below the valves; phosphor bronze adaptors, shrunk, screwed and dowelled in position accommodated them. The threaded boss for the gas starter non-return valve was at the front, slightly on the push rods' starboard side; an adaptor was also provided for this. That gas starter valve threaded portion that screwed into the cylinder head was not normally detached when the valve body was removed. A small tapped blind hole, for connection of a cylinder head temperature thermocouple was provided immediately below the starboard inlet port.

Valves

Valves and Guides

The two inlet and two exhaust valves in each cylinder worked in phosphor bronze valve guides pressed into the cylinder head. Both valves were of K.E. 965 steel, with nitrogen hardened stems. The inlet and exhaust valves were not interchangeable, as they differed in design and dimensions. The inlet valve head and stem were of lighter construction and the face was narrower than the exhaust valve. The head upper surface of both valves was slightly concave. Three concentric springs were fitted to each valve, and the valve spring washers were anchored to the stems with split collars. Three grooves were machined in the valve stem engaged shoulders in the split collars. The collar outer surface was tapered and was received in a corresponding taper in the valve spring washer, causing the collar to grip the valve stem. A wire circlip  fitted in a fourth and narrower groove above the other threer prevented the valve from falling into the cylinder if the valve spring anchorage failed. The valve stem ends were tipped with a steel stud with a hardened head and soft shank that provided a working surface for the hardened steel ball in the valve rocker screw. The stud shank was a press fit in a hole in the valve stem top. The three valve springs were captured between the valve spring washer and a steel seating washer that was a push fit on the valve guide and abutted a shoulder thereon. The washer faces that received the springs were machined with grooves to locate the springs concentrically. The valve guides were of phosphor bronze, made with a parallel-sided shank that was a press fit in its cylinder head socket. Above this portion the guide had a conical shoulder that abutted a corresponding seating in the head, limiting the guide's entry depth in the head. The shoulder's upper face was flat and made, in conjunction with a step on the shank, the lower valve spring washer's seating.

Rocker Gear

Rocker Bracket and Covver	Rocker Dismantled	Rocker Gear on Cylinder Head	Tie and Push Rods	Rocker Assembly Sectioned

Earlier VIII T. engines featured a pair of lugs that were screwed and shrunk into bosses on the cylinder head top and constituted permanent fixtures. These were superseded by bolted-on, platform type lugs, each of which was secured to the head by three studs. Later cylinder heads provided two lugs for attachment of a Townend exhaust manifold. Each cylinder's rocker assembly comprised two sets of concentric rocker shafts. Each set was carried in ball bearings in a rocker bracket and each set operated one inlet and one exhaust valve. The two brackets were parallel to each other along the cylinder head and were joined at the end by short bolted-on I-section bridge pieces that made the assembly a rigid rectangular unit. Each rocker bracket was an open-sided cylindrical forging with a ball-bearing housing at each end to accommodate the inner rocker shaft bearings. Faced bosses on the housing sides were each fitted with two studs for attachment of the bridge pieces, while downward-projecting lugs attached the unit at the rear to lugs on the cylinders and at the front to tie rods anchored to the crankcase. As the unit overhung the cylinder sufficiently to permit the tie rods and the push rods to clear the cylinder fins, the rocker shafts were comparatively long. Both shafts were hollow. The inlet rocker shaft, which was the innermost member of the assembly, extended the length of the rocker bracket and projected at either end to carry on its rear end the rocker arm and on its front end the push rod lever. The shaft was borne in two ball bearings that were housed in the rocker bracket ends. The push rod lever was integral with the shaft and formed the anchorage against which the items assembled on the shaft were drawn up. The rocker arm lug and the end fitting on the rod were bushed with case-hardened nickel steel bushes to receive pins of similar material. A small projection on the tie rod fork acted as a stop that prevented the tie rod from falling forward into the airscrew if the rod's outer end becoming detached. The tie rod lugs' firm attachment to the crankcase was essential; a parallel-sided surface on the lug  shank was machined to fit a corresponding reamed hole in the crankcase face. The lug was drawn into and secured in the hole by a split-pinned nut and washer on the inside. The end fittings for the two lugs adjacent to and on either side of the sump differed from the remainder as they were slightly staggered to clear that receptacle. The upper end fittings on these two tie rods, which were also slightly out of line to match the stagger of the lower ones, had a small rib to distinguish them from the remainder. The bolts at the rocker bracket front were long enough to attach lugs of a rocker cover that enclosed the rocker unit front end and the push rod levers. The bolts were stepped down at their outer ends to receive a plain stiff spring washer. After this the bolt was further stepped down and threaded to receive first a small cap that enclosed the spring washer and then the securing nut. The bolt was therefore secured by the pressure of the nut exerted through the spring washer, the nut being drawn up against the second step on the bolt. The nut was locked with a split pin. The cast aluminium rocker covers were split in the horizontal plane, the two halves being clamped around the rocker unit by four studs fixed in the upper half and projecting through the lower, where they were secured by plain and spring-washered nuts. Two lugs projecting through the floor of the lower half provide for the attachment of the cover to the tie rod bolts. Two rectangular clearance apertures cut in the floor at each end permitted the rocker bracket lugs to project. The push rod entered through two circular holes in the middle of the floor and engaged the push rod levers within. Registers made around these holes received the steel seating washers for Flexekas push rod seals. A swiveling spring-loaded inspection plate was provided on the cover's upper half to access to the push rod lever lubricators within. Two holes in the cover's front wall allowed the lubricators on the inlet rocker shaft front ends to project outside. Two large apertures cut in the rear wall encompassed the exhaust rocker shafts, which project rearwards through these.

The holes in the cover floor through which the rocker unit attachment lugs projected were sealed with rubber composition seatings. Flexekas spring seals captured between collars on the push rods and the seating washers abutted the cover's floor under surface, completely enclosing the push rods' upper portion and thereby screening the aperture by which the rod entered the cover. In earlier engines corrugated rubber seals were used instead of the Flexekas seals. The moving exhaust rocker shaft that projected through the cover rear wall was sealed by a felt washer inset in a diecast silicon ring, made in two halves that were clamped together between the two rocker cover halves. In addition to the two lubricators in the rocker shaft front ends, there were the two other nipples in the push rod lever ends. These were reached by a swiveling spring-loaded rectangular inspection plate on the rocker cover top. The apertures covered by the plate were opened by raising the latter with the fingers and turning it through 90°.

Transverse cylinder head expansion provisions in the early rocker unit attachment was effected by treating the starboard lug on the cylinder as the master lug of the pair and fitting the forked lug on the rocker bracket to this one with no side clearance at all. On the other lug a tolerance of 0.006 – 0.012" was allowed between the inner face and 0.020 – 0.030" between the eyebolt outer face and the adjacent lug face, thereby providing ample tolerance for head expansion. In the platform type lugs a clearance of 0.015" was allowed on the outside face of both lugs, while they were both a close fit on the inside face, thus allowing for the expansion of the cylinder head. The rockers were lubricated by injecting grease with a grease gun. The grease gun nipple was screwed into the front end of a magnesium plug that was a push fit in the inlet rocker shaft bore and which was made with a thread immediately behind its hexagon head to screw into a corresponding thread in the front end of the shaft's bore. From the nipple the grease passed along a narrow bore in the plug that extends halfway along it and terminated in a radial hole leading the lubricant onto a flat extending the plug's whole length, by means of which each bearing received an equal share. From here the grease passed out by radially drilled holes in the shaft co-lateral with the distance rings between the bearings at each end. The rings, which were internally grooved and radially drilled, distributed the grease to the bearings on each side of them. The plug front end was waisted so that the holes in the shaft were not masked; at the rear end the masking was avoided because the plug end was short of the holes. The rocker screw bores were periodically charged with grease by hand.

An important rocker gear feature was the rocker arm's angle of attack upon the valve. An incorrect angle of attack resulted in undue side thrust causing wear between the rocker point and the valve stem top. The side thrust was in turn transmitted to the valve stem and set up excessive stem and guide wear. The most favorable angle of attack occurred when the virtual line leverage (i.e., the line passing through the fulcrum center and the actual point of the rocker that contacted the valve) was at right angles to the valve axis stem when the valve was at approximately one-third lift. The angle of attack determining factor was the rocker fulcrum location in relation to the valve top.

Valve Clearance Compensation

Valve Clearance Compensation

As cylinder temperatures increase, thermal expansion elongates the cylinder while the push rod lengths remain essentially constant, causing the valve clearances to change. The valve clearance compensating gear maintained consistent valve clearance by linking the rocker assembly front end to the crankcase with tie rods so that as the cylinder expanded the rocker assembly pivoted about the foremost rocker bracket pin. At the same time, since the rocker bracket pivotal axis was at right angles to the rocker arm, the rocker bracket angular displacement took place in the longitudinal plane, while the resulting rocker lever angular displacement occurred in the transverse plane. Since the inlet rocker shaft overhung both the front and rear rocker bracket pivot points, uncompensated cylinder expansion caused the inlet rocker to be raised slightly, resulting in an increased valve clearance; however the same bracket movement lowered the push rod at the front end an approximately equal amount, tending to decrease the clearance between the lever and the push rod. The net result was that as the rocker shaft was free to rotate, the changes at each end cancelled out and the total clearance was virtually unaffected. The same principle applied to the exhaust rockers except that since the push rod lever and the rocker lever in this case lie between the pivotal points instead of outside them, the movement of the compensating device would tend to decrease the clearance at the valve and to increase that at the push rod, owing to the fact that the exhaust rocker shaft was between the rocker bracket pivotal points. The slight rocker shaft rotation brought about in the manner described above effected an equally slight change in the rocker angle of attack upon the valves, but this was so small as to be negligible. When the cylinder was cold the rocker bracket was not at right angles to the cylinder axes but was slightly raised at the front end. The bracket assumed a level position when the cylinder was hot and had expanded, thus making the path of the rocker true with the axis of the valve when the engine was in its normal hot running condition.

Cam Gear

Cam Gear Train

Cam Gear	Fig. 26	Fig. 27	Fig. 28	Fig. 30

The cam sleeve and its epicyclic driving gear were situated immediately in front of the crank chamber where they were housed in the front crankcase cam gear housing inner ring. The cam gear assembly was mounted on the fron crankshaft journal. The tappets were arranged radially around the cam gear casing. The inlet tappets were in the forward vertical plane and the exhaust tappets in the aft and parallel plane; each set was operated by a cam disc, a circular plate with four cam lobes projecting from its outer periphery. The two cam plates were integral with a sleeve, bushed at the center to run on its journal and fitted with a driving gear wheel. This unit was known as the cam sleeve. Since the engine firing order involved alternate-firing cylinders (i.e. 1, 3, 5, 7, 9, 2, 4, 6, 8), the tappets also operated in this sequence. The cam rotated opposite the crankshaft direction at 1/8 crankshaft speed. A epicyclic gear train produced the necessary speed reduction and rotation direction reversal. The whole assembly was mounted on the crankshaft except for large fixed annulus known as the fixed internal gear, which was bolted to the crankcase. The assembly consisted of an eccentric was keyed to the crankshaft upon which a pinion freely turned. The pinion had a 68-tooth external spur gear at one end and a 68-tooth internal spur gear at the other. The eccentric's motion made the pinion's  external gear roll around the 72-tooth fixed internal gear, a motion that was transmitted by the pinion's internally-toothed wheel to a 64-tooth spur gearl on the cam sleeve, which was mounted concentrically on the crankshaft.

Figure 26 shows the two cam gear trains (i and ii) separately in plan view, along with the sectioned assembly (iii). Figure 26(i) shows that as the crankshaft and eccentric (A) rotate once in the clockwise direction, the pinion's 68 external teeth (B) will roll around the fixed internal gear's 72 teeth. A's speed increase relative to B will be 72/68 in the anticlockwise direction. Simultaneously, as seen in Figure 26(ii), the pinion's internal gear (C) will roll around the cam disk's 64-tooth external gear (D), which will cause the cam disk to rotate anti-clockwise at a 68/64 ratio. The combined motion of D is (72/68) x (68/64) = 9/8, or 1/8 times crankshaft speed in the opposite direction.

Figures 28 and 30 illustrate the cam assembly. The 64-tooth cam wheel, by which the cam was driven, was pressed onto a spigot and secured to the cam sleeve web by eight close-tolerance bolts in reamed holes. The cam rings were drilled and channeled for lightness. The spur gear ring that drove the gas starter distributor was mounted on four studs on the front cam ring and secured by four split-pinned nuts. The cam sleeve ran on a case-hardened steel sleeve known as the crankshaft sleeve, which was placed onto the crankshaft and abutted the oil thrower in front of the intermediate main crankshaft bearing. The crankshaft sleeve was lined thinly with white-metal to facilitate its passage along the crankshaft during assembly. The cam sleeve was lined with two phosphor bronze bushes that were pressed into either side of the sleeve center. The bushes' internal faces were grooved to conduct oil over the whole surface while an internal groove and holes drilled in the case-hardened sleeve supplied oil to the bearing from an oilway cut on the crankshaft. The groove in one bearing was right-handed and in the other left-handed to conduct the oil away from the central recess; a shallow oil slot was cut down each bearing face at the groove ends to ensure bearing end lubrication. The bearing length was slightly less than the crankshaft sleeve upon which it ran, which allowed the part in front of the bearing to be drawn up tight against the crankshaft sleeve while leaving a working clearance for the cam sleeve.

The next part on the crankshaft was the eccentric, which was milled internally for lightness and keyed to the crankshaft by a feather key. The eccentric bore was lined thinly with white-metal to facilitate its passage along the crankshaft during assembly. A compound pinion known as the cam gear pinion ran on the eccentric's outer surface separated by a floating cast iron bush that was drilled and centrally grooved for lubrication; an oil way cut along the eccentric was supplied from the crankshaft oilways by holes drilled radially through the eccentric. At lip at the eccentric's aft end located the pinion axially, while a ground steel eccentric ring located its front end. The pinion's length was slightly less than the eccentric's, providing end clearance. A concentric extension on the eccentric's front was threaded for attachment of an extractor. The eccentric ring was made with a wide extension flange at its rear, which was drilled for lightness. The ring that was mounted upon the eccentric's extension was slightly longer than the extension to enable the ring to be drawn up with the other fixed components on the crankshaft. Ring rotation was prevented by a locating screw projecting from the eccentric's front face, which entered a hole in the ring flange and was secured by a split pin. In addition to locating the pinion the eccentric ring, which was available in several thicknesses, addressed the accumulation of the tolerance by the items assembled on the crankshaft front end, thus causing the crankshaft to be centrally located in the crankcase. From the foregoing it will be seen that the intermediate main bearing inner ring, oil thrower ring, crankshaft sleeve, eccentric, eccentric ring, front main bearing inner ring,  and crankshaft nut lock washer for the were all fixed items on the shaft, and were drawn up by the left-hand threaded crankshaft nut; for all intents and purposes they were integral with the shaft.

The compound pinion consisted of a sleeve with an external gear machined at one end and an internal gear at the other. The bore was grooved for lubrication. The internal gear overhung the sleeve end to permit engagement with the cam sleeve gear.

The fixed internal gear was a steel plate with an annulus at its center that was secured on the studs that carried the front cover and by a row of studs on the inner tappet ring. A spigot was formed on both sides near its outer periphery; the aft one registered with the crankcase and the front one with the front cover. The holes in the plate through which the studs passed were elongated to enable the timing to be varied by fractions of a tooth, but this adjustment was necessary only in the original engine assembly; thereafter the plate was fixed by two permanent dowels. Lightening and oil drain holes were drilled in the web, and a large clearance aperture at the top permitted the spur wheel on the gas starter distributor drive shaft to engage with the spur ring on the cam sleeve.

Cams Sleeves

Timing Diagrams

Three types of cam sleeves were used in these engines. Workers were directed to note the cam sleeve part number during engine disassembly so that an accurate timing check could be made during assembly. The PN F.B. 17417/3 cam sleeve gave slightly longer valve openings than the PN F.B. 17417, and was introduced to obtain a slight power  increase. The PN F.B. 17417/4 cam sleeve gave gentler tappet acceleration, thereby reducing valve gear loading.

Tappets and Push Rods

Tappets

Case-hardened steel tappets worked in phosphor bronze guides that were mounted in pairs around the cam case. The tappets were hollow and slotted at their inner ends to receive a case hardened steel roller mounted on a hardened pin that was a working fit in both roller and tappet and axially located by the tappet guide. The guide inner end was similarly slotted to receive the roller, preventing the tappet from revolving. The tappet guides were made with a flange at their outer ends and each was secured to the crankcase by two studs. After the tappet was inserted a guard plate was fitted on the studs to prevent entry of foreign matter into the tappet cup. The studs also carried an open-sided flanged cage. A spring captured between the cage top and a collar on the push rod exertsed a downward force on the push rod to help relieve the valve springs of push rod inertial load. Spring-washered nuts secured the assembly on the tappet guide studs. The tappet guides of the six lower cylinders differed from the remainder due to an oil baffle that prevented oil from splashing directly on the tappet and working out due to gravity. Earlier engines applied these baffles to the four lower cylinders only. A trap was formed around the guide lower end and oil collecting there was drained back to the sump by four radially drilled holes in the guide wall. In the front tappet of each pair two of these holes were masked by the crankcase boss walls and the drain holes for these guides were instead interconnected with the rearmost guides by two holes drilled in the metal separating them, thus enabling oil from the front guides to drain into the rear ones and thene to the sump. A small groove machined in the crankcase adjacent to one of the drain holes further assisted oil drainage. Cylinder Nos. 5 and 6 tappet guides differed yet again from the remainder as they were drilled with a number of small downwardly inclined holes around the guide bodies.

The chromium-plated hollow steel push rods were provided with end fittings for contact with the tappet and rocker levers. The fittings were a hard push fit in the rod and no other attachment means was necessary. The fitting at the tappet end was a hollow ball-ended stem, the ball end of which was hardened and worked in the tappet cup. Above the tappet guide the stem had a wide shoulder, after which it was machined parallel to be a push fit in the rod. The shoulder served the combined purpose of forming the abutment for the tappet rod end and the stop for the push rod springs. The gunmetal upper end fitting was cupped at the top to receive the loose steel ball in the rocker lever socket. A hardened steel roller pressed into the cup bottom provided the working contact surface for the steel ball in the socket. Its external surface was hemispherical in shape, so that it took axial location from the rocker lever socket. The length below this the fitting was waisted until it joined the push rod tube, where a collar was formed to provide the abutment for the tube. As with the lower fitting, the shank was machined parallel-sided to be a push fit in the push rod. A collar soldered on the tube a short way from the top provided the stop for the Flexekas sealing spring, which screened off the push rod end.

Rear Cover

Rear Cover

Oil Pump Removed	Looking Aft, Forward

The rear cover providec mounting for the magnetos, hand turning gear, fuel pump and revolution indicator drives, oil pumps and filters, and magneto bevel drive sleeve. The magneto bevel drive sleeve was coupled to the crankshaft rear end with two internal keysthat projected to the sleeve rear and engaged the oil pump driving flange, which lay immediately behind the crankshaft. The sleeve front was made with a bevel gear that meshed with two other bevel gears on the magneto drive shaft front ends. The bevel on the sleeve had 36 teeth, while those on the shafts had 32 in order to provide the required 9/8 magneto speed ratio. The magneto bevel driving sleeve ran in a flanged white metal bearing that was pressed into a housing in the rear cover center. It was pressed in from the front and its flange formed the stop that limited its entry into its housing. A collar formed on the magneto bevel driving sleeve immediately behind the bevel provided rearward sleeve axial location, making working contact with the white metal bearing flange. Forward axial location was obtained by a pair of keep plates mounted on four studs projecting from the rear cover body; they were recessed around their central aperture to receive the collar on the sleeve. The keep plates served also to retain the bearing flange, which was not drilled for the studs but was scalloped to receive steel distance pieces of the same thickness as the flange to fit into the scallops. This arrangement prevented the soft white metal from being crushed by the fastener pressure and distorting the plates, thereby reducing the necessary working clearance.

The two magneto drive spindles lay at 32° to the magneto driving bevel, the included angle between their axes therefore being 64°. Each spindle was carried in a bearing consisting of two flanged bushes each of which was pressed in from one side of the housing provided for them in the rear cover. The bushes did not meet and a space left between carried oil under pressure from the main lubrication system. The spindles were solid and were tapered at their front ends to receive the correspondingly bevel gear tapered bore. The bevels were keyed to the spindles and each was secured by a split-pinned nut. The spindle rear ends had slotted bosses that received the laminated spring magneto coupling dog elements, which were secured by a cross bolt and nut. The spindle was located axially between the bevel rear face and the boss front face, which made working contact with the flanges on the front and rear bushes respectively. The magneto coupling housing, formed by the extended spigots that carried the magnetos, were each provided with two drain holes that conducted drain oil through the rear cover wall and back to the crankcase.

The oil pump and filters were attached to the rear cover lower portion . The magneto bevel driving sleeve open end entered the oil pump chamber, a cylindrical chamber in the rear cover rear face in which the pump was accommodated. The built-up pump body was a complete demountable unit, cylindrical in shape and a push fit in its chamber in the rear cover where it was secured by six studs that passed through a hexagonal flange provided at the pump rear. A rubber ring, compressed between the flange face and a concave chamfer around the chamber mouth, sealed the joint between the pump and the rear cover against oil leakage. The pump chamber axis was below that of the crankshaft. This was convenient for engageing the flange on the pump driving spindle, which was eccentric to the pump body and which engaged the keys in the magneto driving bevel sleeve rear end. The scavenger and pressure filter chambers, which lay to the right and left of, but below, the pump chamber, extend the cover's full depth. Each housed a removable gauze filter mounted on a frame the rear end of which fitted into a waisted hollow steel plug that screwed into and closed the filter chamber rear. Unions for connecting pipes to the filter chambers were mounted in two downwardly-extended elbows that were integral with the rear cover. These communicated with the filter chambers by four holes drilled in the waist of each hollow filter plug. Three oil pipe unions were on the rear cover; the port side one led to the pressure filter, the starboard side one to the scavenger filter and the center one passed the scavenger oil back to the tank. Mounting the unions below the filter itself allowed the filter to be withdrawn for cleaning without disturbing the pipeline. The filter chambers and the pump unit ports communicated via passages drilled in the rear cover below the pump. These passages broke into the pump chamber where in most cases they terminated in a cavity made in the chamber wall. This was necessary because it was not convenient to drill the passage on the same plane as the pump port and the cavities were therefore staggered or stepped backwards or forwards to link up the passage with its appropriate port. A satisfactory joint prevented undue leakage between the respective cavities due to the pump body being a push fit in its chamber, and although a leakage occurred between the cavities, it was negligible. Moreover, the final discharge from the pressure pump was into the chamber itself and the rear end of the chamber was sealed against external leakage. The passage from the pressure filter to the pump chamber was drilled diagonally upwards across the rear cover from port to starboard and entered a cavity halfway along the pump chamber. As this cavity was out of line with the pressure pump inlet port, the cavity was carried rearwards and then upward, the upward extension registering with the pump inlet port. The pressure pump outlet was through the hollow idler spindle, which discharged into the pump chamber where it passed into the crankshaft and to the other passages leading from the chamber, which are described later. The passage from the scavenger filter casing to the scavenger pump inlet port was drilled from the filter casing and broke out immediately opposite the pump inlet port. The pump outlet port discharged into another stepped cavity on the pump casing opposite or port side, the step being necessary because the passage that led from the outlet union to the tank (the center union of the three) was drilled in front of the passage to the pressure pump and was therefore out of line with the scavenger pump port. An annular recess was cut around the magneto bevel driving sleeve white metal bush housing and from this passage oil flowed to the space between the bushes of each of the magneto drive spindle while another upwardly-inclined passage lubricated the hand turning gear. The recess around the white metal bush was fed by two oblong ducts on each side of the bush, which opened into the pump chamber and were fed by the main delivery from the pump. After lubricating the layshaft, the oil flowed out by a swiveling union screwed into a boss immediately in front of the facing from the hand turning gear. A two-way union was fitted; one pipe fed the hand turning gear and the other the fuel pump drive casing. Two holes drilled below the hand turning gear shaft axis registered with two others in the hand turning gear casing and served to drain the oil overflow to the rear cover front and then to the crankcase.

Hand Turning Gear

Hand Turning Gear

Sectioned	Dismantled	Trip Gear	General Arrangement

The hand turning gear was bolted to a vertical face on the rear cover above the oil pump. The hand turning gear shaft or layshaft was carried at its forward end in a duralumin bearing consisting of two plain bushes that were pressed into a housing for them between the facing that carried the gear and the rear cover front face. The annular space between the bushes and the oil passage leading into this space lubricated the hand turning gear and the fuel pump drive mounted at the hand turning gear rear. The layshaft aft end ran in a ball bearing in the hand turning gear casing rear end plate. The layshaft projected through the casing end plate and extended aft to mesh with the fuel pump driving shaft. An 18-tooth spur gear, integral with the layshaft front, engaged a 36-tooth gear mounted on the crankshaft rear. The hand turning gear provided a 10.18:1 speed reduction between the crank handle and the layshaft, which, combined with a 2:1 lay shaft reduction, resulted in a 20.36:1 total speed reduction between the turning gear crank handle and the crankshaft. This speed reduction was effected by a train of bevel gears and pinions that transmitted the drive from the transverse coupling shaft to which the crank handle was fitted to a hollow sliding claw dog shaft that was concentric with and engaged a corresponding claw dog splined to the layshaft. Some engines were fitted with an early gear type in which the reduction was effected by a worm and worm wheel, but the majority of these were replaced by the later type. The gear was housed in an aluminium casing, the main body of which was split vertically, the two halves being face-jointed and located by dowel pins and hollow dowels. The halves were held together by two bolts and six long studs, which also secured the spigoted casing in the rear cover. The two outermost bolts carried the hollow dowels. A spring loaded toggle type trip gear operateed the sliding shaft through a striker fork that acted through a ball thrust on the shaft's rear end. The trip gear, which was engaged by a hand lever, was automatically thrown out of engagement as soon as the engine started by the claw dogs themselves, which were backed off at a suitable angle to effect this. The hand lever was drilled with a hole for attaching a remote control cable for operation from the cockpit. The casing rear end was closed by the foremost wall of the small circular casing, which was spigot-jointed and dowelled to it. The space occupied by this casing was originally designed to house the gun synchronizing gear casing, which was moved to the engine front; the dummy casing, which served as the distance piece to carry the fuel pump driving gear, provided a spigot at its rear end for the fuel pump drive casing.

The first member of the bevel drive was an 11-tooth bevel pinion integral with the transverse coupling shaft that carried the crank handle couplings. The shaft was borne in two ball bearings, one at each end, which were accommodated in the main casing housing. The pinion meshed with a 32-tooth horizontal bevel wheel that was made on a shaft borne in two caged ball bearings, the outer races of which were a press fit in their housings in the casing. The hollow shaft bore was splined to receive a correspondingly splined shank of an 8-tooth bevel pinion, which constituted the second driving member of the train, and which was secured in the shaft by a flat headed screw. Axial location of the horizontal bevel was effected by shims inserted between it and the inner race of its ball bearing. The pinion meshed with the final member of the drive, a 28-tooth vertical bevel wheel whose short shaft was carried in a twin ball bearing pressed into its housing in the casing. This bevel's axial location was effected by a shim inserted between the bearing outer race and the housing bottom. The comparatively large bore of this wheel's shaft was splined to receive the correspondingly splined external surface of the sliding shaft, which was free to move axially therein. The shaft bore was tinned to reduce sliding friction. In addition to being borne in the bevel wheel shaft splines, the sliding shaft was supported at the front in a floating ball bearing of somewhat unusual design. The shaft was stepped-up at this end to provide the necessary diameter for the claw dog to be cut in its end face, and a short hardened steel sleeve, flanged at its rear end, was pressed from the rear onto the stepped-up portion and projected forward to the dog. Sufficient clearance was left between the sleeve end and the adjacent casing to allow for the shaft travel when it moved forward to engage the claw dog on the layshaft. Two rows of balls mounted in a duralumin cage were fitted on the sleeve and separated it from a steel ring, which was pressed into the casing. The ring served as the bearing outer race while the sleeve on the shaft constituted the inner one. The balls were prevented from wandering rearwards by the flange on the sleeve rear end, and forwards by the internal lip at the liner forward end. The sliding shaft was stepped down in two stages to carry the thrust ball bearing for the trip gear striker fork. The foremost bearing thrust ring was a push fit on the first step of the shaft, after which the shaft was reduced by the second step to clear the stationary rear thrust ring, which was a push fit in the ball thrust housing. The housing bore was recessed at the front to clear the foremost thrust ring. The thrust bearing was axially retained in the housing between an internal shoulder at the front end and a threaded ring at the rear; it was retained on the shaft by a nut. The nut and the threaded ring were made with an internal and external lip respectively at their rear ends to limit their threads' entry depth, leaving the thrust ring free, and thereby providing the necessary working clearance for the bearing. The ring and the nut were each locked with spring wire rings.

A pair of trunnions was made on the ball thrust housing outer surface to engage the trip gear striker fork. The layshaft ran through the sliding shaft center. The plain bearing in which it was born at the front end consisted of two bushes, each of which was pressed in from opposite ends of a housing provided for them in the rear cover. An oil space left between the bushes was fed by a restricted oil supply from the engine lubrication system. The shaft's rear end was carried in a caged ball bearing housed in a the gun synchronizing gear dummy casing front wall, which closed the rear end of the hand turning gear casing. The dog fixed on the layshaft that was engaged by the dog on the sliding shaft was mounted on the layshaft immediately behind the plain bearing. The layshaft was splined to fit corresponding splines within the dog bore, which was secured in position by a washer and a nut locked with a split pin. The dog shank front was stepped down to receive the inner race of a deep-groove ball bearing, the outer race of which was housed in a recess in the rear cover at the plain bearing rear end. The flange on the bearing's rearmost bush was accommodated in this recess, and was relieved towards the center to permit free rotation of the bearing inner ring. The ball bearing took the thrust from the trip gear spring loading on the sliding shaft when the dogs were engaged. The dog teeth were backed off to a suitable angle to effect their disengagement when the engine started and the layshaft overran the sliding shaft, and the trip gear was arranged so that the spring loading on the striker fork was thrown over by this movement and the gear was disengaged. The dogs' front or driving faces were also ground to a slightly negative angle sufficient to disengage them under the torque load, were it not for the spring loading of the gear that held them engaged. This provision reduced the effects of a back fire transmitted to the gear and thence to the operator's hand. If a back fire occurred, and the loading on the dogs exceed the pressure the spring was capable of exerting, they were disengaged in the same manner as when the engine started, except that the driving faces, instead of the backs of the dogs, effected the disengagement. Even with this provision, a back fire would be felt severely by the operator's hand, and a ratchet was therefore provided on the crank handle to render its movement irreversible. The ball bearing that supported the layshaft rear end was not axially located in its housing, and was free to take up its location with respect to the front bearing. The rear bearing's inner race was secured by a distance piece, locking washer and nut screwed onto the shaft end. A washer was interposed between the inner race front face and the shoulder on the shaft against which this abutted. An oil thrower was interposed between the inner race rear face and the distance piece. The hollow layshaft front portion was of larger diameter than the rear in order to provide the necessary stiffness for the heavy torque load transmitted by this shaft section. The shaft was hollow throughout and was counterbored in five progressive stages to render its thickness proportionate to its loading along its length. The striker fork that engaged the trunnions on the sliding thrust collar was mounted on a splined transverse shaft that projected beyond it and the adjoining casing at each end. Two steel distance sleeves were fitted, one on each end of the splined shaft, and abutted the fork boss. These were followed by two upwardly disposed levers abutting the distance pieces; the levers were joined at their upper ends by a hollow pin. One end of the splined shaft was threaded to receive a nut that drew up the levers, the distance pieces and the fork boss against a head on the other end of the shaft. The distance pieces spaced the assembled items on the shaft to provide the required end float and freedom for the shaft and they therefore constituted journals that worked in unbushed holes in the casing.

The striker fork levers' hollow pin also passed through a boss approximately one-third up the hand lever. The spring's upward pressure on the fulcrum pin converted the short section of the hand lever between the fulcrum and the upper pin in the striker fork levers into a spring-loaded toggle. In passing through its angular range of movement the hand lever passed through a dead center position on either side of which it was retained by the spring's upward pressure. The vertical component of this movement was permitted by the link's pivotal mounting, which allowed it to follow the path through which the pin was constrained to pass. The link was anchored to a lug bolted to the casing top and the link was pivoted to the lug by attaching it to a rocking pin borne transversely in the lug. The link that passed through a hole drilled diametrically through the rocking pin was stepped up for the latter half of its length, a collar at the rear end of the step providing the stop, which with a distance piece abutted the rocking pin. The link was drawn up hard against the stop by a split-pinned sleeve nut on a threaded portion of its length. A full clearance hole was drilled in the lug and was suitably chamfered at the bottom in front and the top at the rear to permit the link's rocking movement. The link's length  was such that it brought the fulcrum pin vertically in line with the striker fork center, thus providing the correct position for gear throw-out movement. A pair of stops was provided on the lug against which the striker fork levers rested in the disengaged position.

Oil was supplied to the hand turning gear from the oil recess formed between the two layshaft plain bearing halves. An upward passage from the bearing was fitted with a swivel union that carried a two-way piece at its outer end from which one pipe ran to the hand turning gear casing, and the other to the fuel pump drive. As a direct supply of oil from the oil recess would greatly exceed the quantity required by the two units, the feed was rendered intermittent by placing the passage slightly behind the recess (where a hole was drilled in a bush to correspond with it) and providing a flat on the layshaft that interconnected the recess and the passage for a short period during every shaft revolution. The oil line to the hand turning gear entered the casing top and delivered oil to the bevels and flooded the casing to a level maintained by two drain holes that conducted the overflow into the rear gear and thus to the sump.

The irreversible ratchet attachment on the coupling shaft consisted of a ratchet on the end face of a sleeve that was splined to the shaft. The ratchet on the shaft meshed with a corresponding fixed ratchet mounted on the hand turning gear casing. The ratchet attachment was housed in a small circular casing. The ratchet's fixed member was a ring, flanged at the rear for attachment to the casing studs. The starboard shaft end was stiffer than the port end and included the bevel pinion, which transmitted the drive to the bevel below. The bearing at this end was also heavier, as in addition to serving as a journal bearing, it carried the thrust load resulting from bevel pinion and bevel engagement. The larger bearing was housed in a steel-lined housing in the main casing, its outer race being axially retained by a lip at the steel lining's bottom against which it was held by the fixed ratchet member that spigoted into the housing mouth. The bearing determined the shaft's axial location, and a laminated shim inserted between the bearing housing and the casing main body provided adjustment. Three studs projecting from the main casing faced surface around the housing carried the laminated shim, the steel bearing housing, lugs projecting from the flange around the ratchet fixed member, and similar lugs projecting from the ratchet attachment cover. The cover was provided with a felt oil retaining washer at its outer end. The splines upon which the ratchet moving member was borne were not cut directly on the shaft, but on a sleeve fitted on to shaft splines that abutted the bearing inner race, retaining it against a shoulder formed on the bevel pinion back. The race and sleeve were retained on the shaft by another flange sleeve that was secured by the distance piece and cross pin of the universally jointed crank handle socket at the shaft's extreme end. The ratchet loading spring was captured between the flange on the sleeve and a corresponding shoulder on the ratchet. The smaller bearing at the shaft's port end was a sliding fit on a stepped-up portion of it. Its outer race was accommodated in the housing, and was retained therein by a cover plate, fitted with a felt washer and secured to the casing by three screws. The universally- jointed sockets at each end of the shaft that received the crank handle were not called upon to take up extremes of misalignment, and were therefore not designed to provide a very large range of universal movement. The socket bores that received the handle were circular, the handle being keyed and retained in them by a cross bolt that passed through the sockets and handle shank. The universal joint was formed in the socket end in which were cut two diametrically opposed slots. The slots received the end of the cross pin that passed through the coupling shaft end, and the pin was captured in the slots by a cap that fitted over the back of the socket and was secured by screws. While the slot widths corresponded with the pin diameter, they were greater in depth, thus permitting the socket to rock over a limited range in one direction. In the other direction the socket pivoted on the pin. The pin was retained in the bevel pinion shaft by a small tab-washered set screw, which was entered from the shaft end and passed through a hole in the pin.

Fuel Pump Drive and Revolution Indicator Drive

The fuel pump drive and revolution indicator drive were housed in a separate aluminium casing at the gun synchronizing gear dummy casing rear and constituted the final assembly on the layshaft. The unit was situated vertically over the fuel pump, which was mounted on the induction elbow below, and the drive was transmitted to the pump through an external tubular shaft enclosed in a telescopic aluminium casing. Each end of the tubular shaft was fitted with a slotted socket, to form one element of a universal coupling that engaged with corresponding elements on the drive and the pump shafts respectively. The fittings on the drive and pump shafts consisted of spherical members with trunnions that engaged the slots in the socket.  The fuel pump drive casing front wall provided a housing for a flanged phosphor bronze bush (grooved for lubrication) carrying a short horizontal shaft, which was driven by the layshaft through a tongue-and-slot coupling; the shaft rear end was borne in the casing cover plate of the, where another flanged bush was pressed into a housing . Two spiral gears were machined on the shaft. The front and larger one provided the fuel pump drive and located the shaft axially by making working contact with the front bush flange; the rear one drove the revolution indicator drive shaft. A collar made on the shaft rear gear worked against the rear bush flanged end and completed the shaft's axial location within the casing. Casing extensions left and right of the layshaft axis housed the vertical revolution indicator drive spindle and the fuel pump drive spindle.

The revolution indicator drive spindle was mounted in a long duralumin bush, which had one flanged and one plain spigot on its outer surface to secure and locate it in a reamed aperture in the casing top; the bush was secured by two set screws that passed through holes in the flange. The bush external upper end was threaded to receive the flexible drive shaft casing union nut , and the spindle was bored and internally squared to receive the flexible drive shaft fittings. A collar at the shaft upper end made a working surface for the duralumin bush and located the shaft at the upper end; the worm wheel, which was pressed onto the shaft and secured by pinning and soldering, located the shaft's lower end.

The fuel pump drive spindle was carried in two phosphor bronze bushes. The upper bush was pressed and pinned into a cover cap that spigoted on the drive casing top and was secured by two studs and spring-washered nuts. The lower bush was in two parts that were pressed into each end of the boss provided for them in the casing. A gear wheel formed integrally with the spindle towards the upper end engaged a spiral gear and horizontal shaft, and was located between the upper and lower bush flanges, which located the spindle. The spindle lower end projected outside the casing and was formed with the spherical coupling  for engagement with the external tubular shaft. The trunnions, or cross-pins, in this coupling were a light drive fit in the spindle hole and was secured by small cotter pin; it was necessary to remove both pins when  the spindle was removed from the casing. A swiveling union fitted in the cover plate top at the pump drive spindle upper end connected to the oil feed pipe from the intermittently-fed union at the layshaft front end. The pipe was supported midway by a bracket attached at the hand turning gear casing side. The gear oil bath level was maintained at the casing bottom by an external steel overflow pipe that drained surplus oil back to the rear cover bottom. A threaded boss formed around the casing lower end received a large lock-wired clamping nut by which the telescopic casing upper end was held in position.

Fuel Pump

Fuel Pump

L: Dismantled R: Assembled	L: Dismantled R: Assembled	Sectioned

The driving spindle had a square head at its lower end that engaged a square hole in the driving pinion bottom; the pinion bore was not carried through to its lower end, a wall being left in which the square hole was cut. The square shaft head was a loose fit in the pinion, thus providing a form of floating coupling that promoted spindle and spigot alignment. The spindle's entry depth in the pinion was limited by a collar immediately above the spindle square portion. A hardened steel tip was spun into the spindle lower end and worked against a hardened steel ball in the pump housing facing, providing resistance to wear caused by any downward shaft load. The driving spindle upper end  was tapered, fitted with a key and threaded to receive the coupling that connected the spindle to the external tubular drive shaft. This part was similar to the one on the fuel pump drive spindle, except that as it was detachable the trunnions were integralt. The securing nut on the spindle end was locked with a split pin.

The gland housing and spindle bush were interlocked by a tab washer fitted between the bush hexagon and the castellated housing head. Two washer tabs were bent upwards to engage two bush flats while three other tabs were bent downwards to enter castellations on the housing head. The washer was prevented from turning by a long lug on the washer extending down to the pump body to which it was secured by a spring-washered screw. A duct in the pump body drained any fuel that leaked upwards past the gland. The duct outer end was fitted with a union for connecting a drain pipe. The spindle upper-end bush and gland housing were enclosed in a small domed casing jointed to the pump main body by a paper washer, four studs and spring-washered nuts. A grease gun nipple on the casing supplied the gland with grease. A short externally-threaded extension on the dome tip received a large union nut that clamped the vertical drive shaft casing lower end to the dome; the nut was drilled to receive a locking wire, which was threaded between it and a lug provided on the dome. A valve in the pump chamber roof released air from the chamber if an airlock formed. This valve was a small screw with a flat along its length, fitted with a fiber washer between its head and the boss into which it was screwed. When the screw was slackened air was released along the flat. When screwed tight, the valve was sealed by the fiber washer. The screw was normally locked with wire.

Two phosphor bronze facings covering the area that contacted the pinions protected the aluminium cover plate at the pump bottom from wear by the pinions. The facing beneath the driving pinion was a plain disc with a small spigot beneath, which fitted into a socket cut in the cover; a hardened steel ball was spun into this disc's top surface immediately below the driving spindle. The driven pinion facing consisted of a flange formed toward the lower end of a hollow phosphor bronze stub upon which the driven pinion ran; a short portion of the stub projected below the flange and entered a recess in the cover, while at the upper end the stub entered a socket provided for it in the pump casing roof.

The pump was lubricated by the fuel circulating through it and for this purpose a lubrication groove was cut down the stub's length to distribute the fuel over its surface; a small radial hole drilled in the pinion passed a small quantity of fuel into the pinion bore and provided the necessary lubrication supply. The driving pinion was lubricated by a radial groove in the phosphor-bronze facing beneath the pinion, which led fuel from the pump delivery side into a circular groove around the facing center. A small hole in the solid wall at the pinion bottom coincided with this groove and passed sufficient fuel to adequately lubricate the pinion and its stub. Since the fuel passing through the pump provided lubrication, it was essential that the pump never ran dry. Hence, the fuel systems of aircraft incorporating these pumps were designed to keep the pump lubricated at all times. The bypass ensured lubrication when the pump supply was cut off, when changing over from one main tank to another, or when the main tanks were exhausted and the gravity tank was feeding the engine. In these circumstances, were it not for the by-pass, the pump, pumping both air and fuel away from itself would run dry, but the bypass permitted sufficient fuel to trickle back from the delivery pipe to the pump intake side to keep the pinions adequately lubricated. The bypass effect on the pump output was negligible. When the aircraft was out of use for any appreciable length of time and the fuel in the pipe line may have evaporated, or when the fuel lines were disconnected, it was essential that the system be primed by hand before the engine was started.

Fuel Relief Valve L: Dismantled; R: Assembled

Any fuel system installation was to employ a relief valve in which the cockpit fuel control connected the fuel pump delivery to the carburettors in order that in these circumstances the flow was maintained at a constant pressure. The valve was adjusted to operate when the pressure exceeded 2 psi; the valve overflow was passed pack to the pump inlet side. The relief valve consisted of a disc-type valve housed in a cast aluminium casing. The body or lower part of the casing was approximately cylindrical and contained the valve, valve spring and spring adjustment. It was closed by the casing upper part and was made with a straight through passage across the top. The passage was fitted with a union and tab washer at each end for insertion of the unit in the delivery pipe line. The two casing portions were spigot-jointed and the joint was sealed with a paper washer and jointing compound; the relief valve parts were attached to each other by six spring-washered screws that passed through a flange formed around each part.

The upper cover inner surface was recessed to accommodate a circular brass plate, which provided seating for the valve. A central orifice in the plate was surrounded on the lower side by an integral seating ring, the face of which was ground to provide the valve seating surface. A port was cut in the straight-through passage that ran across the top casting and this, coinciding with the seating aperture, passed excess fuel from the passage, through the valve and then to the return outlet in the casing body. The valve itself was a hexagonally-shaped steel disc with rounded corners, drilled with a ring of eight 0.281" holes about halfway between its center and circumference. Its upper face was ground to seat on the brass surface in the cover cap, coming in contact with that portion of the surface enclosed within the ring of holes, so that when the pressure opened the valve the fuel was able to pass freely through the holes into the chamber below. A register turned on the valve's lower surface located a large-diameter phosphor bronze spring captured between the valve and a flat brass plate also made with a similar register. The plate was formed at the top of a threaded shank that was integral with it and which constituted the adjustment screw for regulating the spring pressure; the plate thus virtually formed a large flat head to the screw. The shank was screwed through a threaded hole in a central boss in the valve chamber bottom and projected outside the chamber; it had a square at its end to enable spring pressure adjustment. Once set, the adjustment screw was locked by a lock nut. The chamber boss outer surface was threaded to receive a hexagon-flanged screw cover cap that enclosed the adjustment components. The valve lift extent was limited by a stop consisting of a steel stem screwed into the cap bottom. The adjusting screw was bored to receive the stem, which projected above the screw head and thereby limited the valve's downward travel. As the cap was screwed fully home on its boss irrespective of the adjusting screw setting, valve travel was constant although the pressure could be adjusted. The valve chamber interior was machined circular and formed a guide for the valve, the corners of the hexagon of which were rounded off for this purpose. Surplus fuel that forced its way past the valve into the valve chamber left the chamber by a union, similar to those at the top, that was screwed into a boss in the chamber side and provided for connecting the return pipe to the pump intake side. A semi-circular bracket on one side of the casing had three bossed holes for attaching the unit to its mounting.

Propeller Reduction Gear

Propeller Reduction Gear

Longitudinal Section	Casing	Airscrew Shaft and Bevel Pinions	Bevel Wheel Assemblies	Spherical Thrust Ring	Airscrew Shaft Thrust Assembly

The reduction gear was housed in a cast aluminium alloy casing, spigoted to the crankcase front cover and secured by 18 bolts passing through the flanges of each part. The casing was reinforced by internal helical stiffening ribs, formed on its inner walls to resist the torque reaction imposed by the gear mounted at the casing front. The assembly comprised a complete demountable unit that could be removed from the engine by releasing the 18 nuts that secured it to the front cover. The gear train that provided a 2:1 speed reduction between the crankshaft and airscrew shaft was of the bevel wheel and bevel pinion type, which had two opposed bevel wheels, one of which was anchored to the gear casing and the other to the splined crankshaft. Three bevel pinions running on three equally spaced stubs radiating from the airscrew shaft were situated between the bevel wheels and engaged both. The crankshaft rotation with its bevel wheel caused the three stubs and therefore the airscrew shaft to be carried around at half crankshaft speed. An important design feature was equal load distribution over the three pinions. This was obtained by floating the bevel wheel mountings instead of rigidly attaching them to their respective mounts. The range of movement was very limited but provided sufficient freedom to permit equal load distribution.

The short but massive airscrew shaft was machined from a nickel chrome steel die forging. It was borne at the rear end by a spigot that projected beyond the shaft end and entered the crankshaft bore, where it ran in a white-metalled phosphor-bronze bush pressed up to a step in the bore. The bore step was cut with four slots to receive an extractor for bush extraction. A deep-grooved caged ball bearing at the shaft front served the dual purpose of a journal bearing for the shaft and the thrust bearing by which the airscrew thrust was transmitted to the engine. A shoulder made on the shaft immediately behind the bearing inner ring provided a stop for the ring, which abutted a distance piece mounted on the shaft between it and the shoulder. The shaft ahead of the bearing was cut with a left-hand thread to receive the thrust nut that drew the ring up to its stop; the nut was locked by a tab locking washer, the tabs of which engaged castellations oin the nut and slots on the shaft. Forward of the thrust nut the airscrew shaft projected outside the reduction gear casing and its surface was machined to a taper and was cut with serrations to receive the airscrew hub. The shaft slowly tapered to a smaller diameter at the front end where a thread was cut to receive the airscrew hub nut by which the hub was retained on the shaft.

The shaft was bored throughout its length; the bore section from the front end to the bevel pinion stubs was much larger than the rear section, where the bore was considerably reduced until it terminated at the rear end where it was bell-mouthed. The bore front end was blanked off with a threaded plug. Towards the rear end approximately midway between the front and rear bearings the three equally-spaced bevel pinion stubs radiated from the shaft. As with the shaft, the stubs were bored from the stub ends to the root after which the bore diameter was reduced to form an oil passage. The bore outer end was threaded to receive a plug that closed the stub end and also secured the locking washer by which the bevel pinion thrust nuts were retained. A passage in the stub at right angles to its bore led oil from the bore to the pinion stub outer surface; the passage broke on to a distributing flat that covered most of the stub length. The major portion of the stub exterior surface was parallel-sided to form the bevel pinion journal. At the inner end a shoulder provided a working surface that located the pinion axially, while at the outer end beyond the pinion journal the diameter was stepped down to accommodate the thrust nut of a thrust ball bearing assembly that received the pinion's outward thrust. Additional rigidity was given to the stub roots by a stiff circumferential web linking their roots; this was made by machining away the metal on either side of it.

The airscrew shaft external diameter increased aft of the stubs. A shoulder just short of the rear end anchored the rearmost bevel wheel ball bearing thrust ring; the extreme shaft end was threaded to receive the thrust nut for this bearing. An annular recess in the shaft rear end accommodated the crankshaft hollow end. An aft-extending spigot projecting from the recess center entered the bush in the crankshaft bore, which served as the airscrew shaft rear bearing.

The bevel pinions were made with housings at their centers for phosphor-bronze white-metal lined bushes, which were pressed into them from the inner end; the bushes were flanged at their inner ends to provide a working surface with the stub shoulders. A ball thrust bearing was mounted in the outer pinion end and was separated from the fixed bearing ring by a row of caged balls. The fixed ring was secured and located by a thrust nut that engaged the external thread on the stub end, and was screwed down to the shoulder at the thread end. Pinion end clearance was obtained by inserting a shim of specific thickness between the thrust nut and the shoulder. The thrust nut was locked by a tabbed locking washer whose outer circumference tabs could enter the spanner sockets on the nut's surface; one tab on the inner circumference engaged a slot in the stub thread. The plug in the stub's end was locked by a third tab on the washer.

The bevel wheels had floating mountings to compensate for pinion load variations. The stationary front bevel wheel was coupled to the reduction gear casing by a splined anchor ring, while the revolving rear bevel wheel was coupled to the crankshaft by a splined carrier. Internally splined rims at the bevel wheel backs engaged with their respective coupling members, but the fit between the rims and the coupling member external splines was sufficiently slack to permit both free bevel wheel axial movement, but also a certain-amount of lateral freedom. The large bevel wheel central apertures had sphericl sections that seated on spherical thrust rings mounted on the coupling members; these constituted the lateral bevel wheel locating members. The fixed coupling member, which formed the anchorage against which the gear operated, was spigoted into the central aperture in the gear casing front, where it was retained by nine waisted studs that projected through the casing wall and were secured by split-pinned nuts on the outer face. The studs were mounted in a ring on the anchorage; the metal between the studs was removed leaving them mounted on integral blocks that engaged with corresponding slots on the casing wall rear face and locked the anchorage against rotation. Behind this ring the splined ring that engaged the front bevel wheel was formed into an anchorage. The anchorage rear face had a narrow spigot on which a spherical-sectioned phosphor-bronze thrust ring, drilled with oil holes, was mounted; it was upon this ring that the bevel wheel seated.

The airscrew shaft main ball bearing, which received the airscrew thrust, was housed in the anchorage bore, its outer ring retained by a threaded ring that entered the bore; the ring's front face was drilled with lightening holes, which also served for spanner engagement and for receiving a lock washer's inner tabs. The ring, which was threaded both internally and externally, received in its bore the threaded spigot of an annular cover plate that closed the housing front. The cover plate and ring were locked by the lock washer, which coupled them together and was itself secured to two housing slots. The plate's inner surface was faced to make working contact with a spring-loaded oil-retaining L-sectioned phosphor bronze gland ring, which was kept in contact with the plate by four small helical springs captured between it and the thrust locking washer.

The bevel wheel carrier by which the rearmost and revolving bevel wheel was coupled to the crankshaft consisted of an externally-splined rim joined by a dished web, drilled with lightening holes, to a tapered central hub. The hub was internally splined to engage corresponding crankshaft splines and was drawn up and retained on the shaft by a ring nut locked by a tab washer. An extractor thread was cut on the hub surface. The rearmost bevel wheel's mounting was complicated by the fact that this wheel was revolving in relation to the airscrew shaft, which projected through its center; a single-row caged ball thrust bearing between the two to received the load. The rearmost ring of this bearing was clamped against the shoulder at the airscrew shaft end; it was separated from the foremost ring by the row of caged balls. A spherical-sectioned steel ring spigoted into the bevel wheel rear face seated on the foremost ring with its spherical-sectioned seating on its outer face. The steel ring was drilled and grooved to ensure a plentiful oil supply reached the contacting surfaces. The ring and cage bores were larger than the rearmost ring in order to clear the airscrew shaft. Pinion lash was obtained by progressively adjusting the gear from front to rear, the main bearing housing for the fixed anchorage being regarded as the base. The bevel pinions were positioned in relation to the foremost bevel wheel by the axial location of the airscrew shaft which was obtained by inserting a distance piece of suitable thickness between the main bearing inner ring and a shoulder on the shaft. The rear bevel wheel was positioned in relation to the pinions by selection for thickness the front face of the spherical-sectioned seating ring that lay between the crown wheel and the ball thrust bearing foremost ring.

The airscrew shaft main ball thrust bearing received the airscrew thrust. The tractor thrust load passed from the shoulder on the shaft, through the distance ring, the bearing inner ring, the balls, the bearing outer ring, the threaded bearing retaining ring the in the housing mouth, and then through the housing and reduction gear casing to the engine. When the direction of thrust was reversed the load passes through the thrust nut on the shaft, the inner ring of the bearing, the balls, the outer ring of the bearing, and thence through the housing and the reduction gear casing to the engine.

Lubricating oil was supplied to the reduction gear from the crankshaft front end bore, the rear end of which had a baffle that reduced oil flow to meet the gear requirements. As oil passed along the crankshaft bore a certain quantity lubricated the airscrew shaft rear bearing, which was housed in the bore front end; the bush was grooved to distribute the oil. After lubricating the bush the oil flowed from the crankshaft front end into an annular recess at the airscrew shaft's aft end. Two small holes led oil, by centrifugal force, to the ball thrust bearing and the spherical ring. The main flow from the crankshaft filled the airscrew shaft bore. A passage drilled in each stub at right angles to its axis carried oil from the stub's hollow bore to an oil flat on its working surface, providing lubrication for each bevel pinions. Drain oil collected in the reduction gear casing where it was maintained by an overflow pipe at the requisite level to provide an oil bath in which the gear dipped; the overflow pipe returned surplus oil to the engine sump. The pipe was in two pieces, joined by a hose connection and clips; wire gauze was wrapped around the rubber to provide the necessary bonding, and each clip was connected to its respective portion of the pipe by a short length of flexible wire. The remainder of the reduction gear, including the thrust bearing, was oiled by splash. Two drain holes in the thrust housing lower segment and two grooves in the casing registering with these drained surplus oil from the bearing.

Gas Starter Distributor.

Gas Starter Distributor

L: Dismantled R: Assembled Inset: Supply Union and Filter	Drive	Sectional Elavation Piping Arrangement	Cylinder Check Valve L: Assembled R: Dismantled

The gas starter distributor was mounted at the front cover top, where a faced aperture received the distributor body's flanged spigot. The flange was drilled with four holes to receive four studs on the facing and secured on the studs by spring-washered nuts. The distributor body lower end had another smaller spigot, which was located in an aperture in the roof of a small box-like chamber, cast integrally with the front cover, which housed the distributor drive. The drive consisted of a spur gear ring on the cam plate that engaged a small spur gear wheel at the rear end of a short longitudinal shaft mounted in the chamber. The shaft, which was hollow, was borne at the rear in a phosphor-bronze bush pressed into the chamber rear end, and at the front in a ball bearing, the outer ring of which fitted into a duralumin housing in the chamber. The outer ring was held in position against a shoulder at the housing bottom by the spigot of a flanged spigoted cover, which fitted into the housing mouth and was secured by its flange on two studs with nuts. A laminated shim that fitted between the bearing housing and the chamber front face was selected at assembly to axially locate the bearing and shaft. This accommodated variations in crankshaft and cam gear location that varied in each engine. Sufficient axial tolerance was provided at the rear phosphor bronze bush to permit the necessary adjustment by a shim. Midway along its length the shaft was fitted with a worm that was machined on a sleeve coupled to the shaft by a slot and dog. The sleeve fell short of the spindle end, where the ball bearing inner race was mounted on a shaft projecting from the spindle, and was drawn up against the sleeve by a split-pinned nut and washer on the threaded spindle extremity. At the spur wheel end the sleeve abutted a stepped-up shaft portion that formed the journal for the plain bearing in which the shaft was borne. A slot was machined in the journal to receive the tongue on the sleeve. A worm engaged a worm wheel keyed on the taper at the distributor spindle lower end and was secured by a split-pinned nut. The distributor-to-crankshaft speed ratio was 1:2.

The phosphor-bronze distributor body was cast with a shallow flat block at the top, which was recessed to accommodate a steel rotor. The recess was closed by a threaded aluminium cover cap that converted the recess into a pressure chamber. The joint between the cover cap and the body was made with a copper washer and the cover cap was secured in position by a lock wire passing through one of several small holes provided around the cap outer edge and anchored to one of the distributor pipes. The recess floor was ground to form a track for the steel rotor; nine ports in the track communicated, via horizontal ducts, with nine tapped holes for the delivery unions. The delivery unions, each of which was numbered with the cylinder it served, were arranged in two rows of four and five respectively on either side of the block. A larger hole at one end of one row accommodated the inlet union by which the mixture entered the distributor. The passage from this union was upwardly inclined and passed the mixture into the pressure chamber. The union incorporated a fine-mesh gauze filter to prevent the entry of foreign matter, which might damage the track.

Although the distributor outlet bosses were arranged in consecutive order, the rotor, in making its revolutions at half crankshaft speed, supplied gas to alternate cylinders, as was required by the firing order. This was obtained by arranging the distributor block ports in two semi-circles, of which one served the odd-numbered cylinders and the other the even-numbered ones. The rotor was had two diametrically opposed turning ports, and of these one was spaced nearer the center than the other, so that each served one semi-circle of ports in the block while the other was covering the blank semi-circle. The priming ports in the rotor were designed to operate in the same manner. A stop was fitted in the rotor chamber cap top to limit the rotor spindle upward travel.

The rotor was mounted on a taper on a separate spindle at the distributor spindle upper end and was there secured by a split-pinned nut and washer. The spindle was a sliding fit in the main spindle's hollow bore and a flat on it allowed air to escape when it was slid into position. The main spindle extended down the full length of the distributor body and was borne in a duralumin bearing at its lower end and a ball bearing at its upper end, where the spindle was slightly stepped up to receive the ball bearing. The bearing outer ring fitted into a recess in the faced track center. The main spindle was a clearance fit in a bore drilled throughout the distributor trunk's length. A spiral oil groove machined up the bore's length provided upper bearing lubrication by oil drawn up via spindle rotation. The main spindle upper end had a recessed collar across which was cut a slot that received corresponding dogs on the rotor spindle and coupled the two together. The main distributor spindle was located axially by a collar at the upper end abutting the ball bearing inner ring, and the worm wheel upper surface at the lower end formed a working contact with the duralumin bush flanged end in which it ran. The rotor, however, was not definitely located, but was free to move axially in the main spindle bore, thus permitting it to ride free of the rotor track when the distributor was not in use. The slot depth and dog coupling between it and the main spindle was sufficient to permit this movement to the degree limited by the stop provided in the cover cap center. As was usual in this distributor type, the rotor was brought into contact with its track by incoming gas pressure on its upper surface and maintained comparatively gas-tight contact between its lower surface and the faced track upon which it ran.

The rotor ports through which the mixture passed to the cylinders on firing strokes and causes the engine to turn, were elongated sufficiently to cover two distributor track holes in order to ensure continuity of the turning sequence. The ports were cut through the rotor thickness and were open to the pressure chamber interior. The priming ports were no longer used and in old rotors they were blanked off ; in new ones they were not cut at all. On leaving the distributor, the mixture was carried by nine 1/4" diameter copper pipes to the non-return (check) cylinder valves. The pipes were secured in clips mounted on studs screwed into the main crankcase bolt front ends. The non-return valves in the cylinders were of the usual spring-loaded type. The valve was in a separate housing that screwed into an adaptor screwed into a cylinder boss. The pipes were connected to the valve housing by a swiveling union that was threaded on to the housing stem and clamped against a housing shoulder by a threaded cover cap that screwed onto the opposite end; copper washers were fitted above and below the union to provide a gas-tight joint and similar washers were fitted between the housing and cylinder adaptor. A special eccentrically-bored union was fitted to cylinder No. 5 to enable it to clear the rocker bracket tie rod.

The distributor drive was lubricated by oil thrown from the reduction gear. A semicircular sheet aluminium oil baffle was mounted on four studs at the front cover front to shield the breathers from excessive splash oil from the reduction gear. This plate had an oil chute at the top and oil collected therein flowed through three holes in the plate to enter a smaller chute on the other side, which delivered the oil around the distributor lower spigot. A groove around the spigot top in conjunction with the spigot aperture bore made a circumferential oilway, and two holes drilled radially through the spigot passed the oil into an internal recess surrounding the main spindle from where the oil was carried up to the upper bearing. Two drain holes beneath the bearing returned the oil to the front cover. Two grooves cut through the spigot admitted oil to the drive chamber interior where its level was maintained by an oil drain hole in the chamber back.

Airscrew Hub

Airscrew Hubs Standard and Wide	Standard Hub Sectioned

Bristol offered a standard airscrew hub of normal width between the flanges for use with two-bladed airscrews and another with a dished front flange, or nave plate, to accommodate the wider boss of four-bladed airscrews. Both designs were similar.

The integral airscrew hub rear flange and barrel were machined from a drop forging. The flange joined the barrel in a sweeping radius at both the back and front. A rearward aft barrel extension with an external stiffening collar strengthened the barrel where it was forced onto the airscrew shaft splines. The rear hub bore was tapered and machined with serrations that matched the airscrew shaft serrations. The forward hub was of lighter construction. The hub bore forward end had an internal shoulder, the front face of which was conical to receive a phosphor bronze collet mounted on the shaft and against which the airscrew hub locking nut drew the hub onto the shaft and secured it. The nut had an extension sleeve that extending the hexagon beyond the barrel front, which rendered it accessible with a spanner. It was locked in position by a tongued locking plate that engaged serrations on the barrel end. The locking plate's hexagonal aperture fitted over the nut and was secured by two lugs covering two nut flats. A cross bolt passed diametrically through the airscrew nut and was held in place with a castellated nut and spring pin. The airscrew nut was aluminium bronze, a material adopted to prevent its seizure on the airscrew shaft.

A left-hand threaded screw ring that was screwed down to a stop in the barrel front was used to withdraw the hub from the shaft. The ring formed an abutment with which an external shoulder on the nut engaged, so that hub was withdrawn by unscrewing the nut. The left-hand threaded screw ring was not secured by any locking device as the hub rotation tended to tighten it in the barrel. The nave plate or hub front flange engaged parallel serrations cut on the barrel outer surface and was clamped to the airscrew by a slotted clamping nut that engaged an external thread on the barrel. The nave plate inner faces and the rear flange were made with radial serrations to grip the airscrew boss and assist in preventing fretting between the airscrew and hub. The clamping nut was locked in position by a tab washer. Ten spigoted steel bolts passed through the flange and airscrew boss, and were secured by spigoted split-pinned nuts in front. In the standard hub the nave plate was flat, but in the wide hub a dished nave plate gave the additional width for the four-bladed airscrews in which the hub was of greater thickness. Apart from the provision of the dished nave plate the spline length upon which the nave plate was mounted on the barrel was sufficient to provide considerable latitude in airscrew hub thickness, and a distance sleeve was available for insertion between the internal securing nut and the nave plate so the nave plate could be clamped down on airscrews with thinner bosses.

A Hucks starter claw was fitted on Jupiter VIIIF. and VIIIF.P. engines but not on XIF. and XIF.P. engines. As a spinner was never fitted, a short type of claw was employed. With the standard narrow hub the claw was made integral with a flanged barrel that was spigoted to a register provided in the nave plate. It was secured to the plate by ten studs and split-pinned nuts. With the wide hub a standard A.G.S. claw was used, and this was mounted on a five-armed spider carried on five of the ten airscrew hub bolts. The Hucks starter was eliminated on later engines.

Gun Synchronizer Gear Generator

The gun synchronizing cam ring fitted into a spigot ring on the airscrew hub rear flange rear face and received the gun synchronizing gear cam supporting plate, which was secured by five small studs projecting from the flange. Ten large holes in the plate cleared the airscrew hub bolts heads.

Two gun synchronizing gear generators could be mounted on two faced platforms cast integral with the reduction gear casing bottom. The platforms were staggered relative to each other to provided the required location for each generator with respect to its cam ring. Only the starboard generator was fitted on VIIIF. and VIIIF.P. engines, and no generator was fitted on XIF. and XIF.P. engines. The generator was carried in a generator securing bracket, a duralumin casting secured to the reduction gear casing  facing by four studs and spring-washered nuts. The generator axis was inclined at a 30° angle of to the crankshaft center line and the cam ring on the airscrew hub back was therefore formed at a complementary angle to this, squarely engaging the roller projecting from the generator casing. The generator was axially located in its split securing bracket by a rib on its outer surface that engaged a corresponding circular groove in the bracket. It was keyed against rotation by a slot in the bracket in which a key on the generator fitted. A grease gun nipple screwed into the bracket bottom allowed grease to be fed into the generator. The cam supporting plate, mounted on the airscrew hub back, was of standard design and provided accommodation for two cams, but as only one generator was fitted the space for the foremost cam was taken up by a distance piece. The cam assembly was secured at the rear by an annular ring mounted on five studs projecting from the cam supporting ring; split-pinned nuts secured the retaining ring in position. The usual ring fine adjustment setting of thirty-six half holes in the cam and forty in the supporting plate rim was provided; small round keys were inserted in the four half holes that matched and formed complete holes when the desired setting has been determined, thus locking the two elements together.

Rear Details	Installation Diagrams

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