Bristol Jupiter IV
Specifications, Description
Compiled by Kimble D. McCutcheon
Published 8 Mar 2025; Revised 14 Mar 2025

Bristol Jupiter IV

The Bristol Jupiter IV was a normally-aspirated direct-drive air-cooled radial 9 that first ran in 1918. Designed by Roy Fedden while he worked for Brazil Straker and later Cosmos Engineering, it featured three carburettors, each one feeding three engine cylinders. Cosmos Engineering went bankrupt in 1920, and was eventually purchased by the Bristol Aeroplane Company. Although the Jupiter was the first air-cooled engine to pass the Air Ministry full-throttle test, the first to be equipped with automatic boost control, and the first to be fitted to airliners, it also became the poster child for poor air-cooled cylinder design; it featured a poultice head, which cooled very poorly due to its joint between the steel cylinder top and aluminum cylinder head, which severely limited exhaust valve cooling and engine reliability. Despite its initial shortcomings, the Jupiter series was continuously improved and built in profusion, powering over 200 aircraft types. It was also licensed in fourteen countries. More than 7,100 Jupiter engines and variants were built. This article addresses an early version, the Jupiter IV, which appeared in 1926. [Wikipedia] Thanks to Bruce Vander Mark for providing the manual from which the majority of this article came.

Specifications

The Bristol Jupiter IV, with a 5.75" bore, 7.50" stroke, 1,752.79 in³ displacement and 4.9:1 compression ratio, had a 400 hp at 1,575 rpm normal rating and a 430 hp at 1,750 rpm maximum rating; its dry weight was 812 lb. Propeller rotation was anti-clockwise when viewed from the anti-propeller end.

Bristol Jupiter IV

Description

Crankcase

Crankcase				Oil Sump	Engine Mounting

The cast aluminum alloy crankcase comprised two castings joined at the cylinder center line and secured by nine close-tolerance bolts, which registered the two halves at the joint. The bolts were formed with collars at their rear ends that were sunk in recesses in the crankcase rear wall. An oval-sectioned annular induction chamber at the crankcase rear was formed partly in the crankcase and partly by an annular cover. Around the chamber's crankcase portion were nine apertures faced to receive induction pipe flanges that were secured by four studs each and which radiated to the cylinders. Three ports in the lower cover segment were faced to receive the carburettor assembly, which was attached by four studs and nuts at each aperture. A circular three-start spiral distributor within the casing distributed the mixture from each of three carburettors to the three cylinders served by each carburettor.

Nine faced surfaces, equally spaced around the crankcase, received the cylinders, each of which was secured by eight waisted studs, four in each crankcase half; the studs, located in pairs at the cylinder base flange corners, were screwed into the crankcase and were further secured by nuts on the inner ends that were locked in place by peening over the stud tops.

The crankcase housed the rear main crankshaft roller bearing in its rear wall and the intermediate main roller bearing in its front wall. Both hearings were carried in flanged steel housings that were a pressed into their bores and bolted to the crankcase walls by eight nuts and bolts. These housing flanges were further secured by a locating piece consisting of a hollow bolt, the head of which was formed into a flat plate ground to a true edge on one side. The edge mated with a flat on the housing's octagonal flange; a lobe on the plate keyed with a corresponding recess in the housing.

Two concentric rings cast integral with the crankcase front face and projecting forward accommodated nine tappet guides and tappet pairs. The outer ring was strengthened by webs between cast between it and the crankcase wall; the cam gear was housed within the inner ring . Nine pairs of faced bosses around the outer ring received the tappet guide flanges. Each boss included two studs and carried a tappet guide, a guard plate to prevent foreign matter from entering the tapped cup, and a heavy four-lugged bracket in the center that secured the tie rod, a temperature compensating element of the valve gear. Machined apertures between the second and third and the eighth and ninth tappets carried two small brass crankcase breathers.

The crankcase front was closed by a front cover bolted to the outer ring by eighteen studs and nuts. The thrust bearing was carried in a steel housing bolted to the front cover by nine bolts and nuts. The crankcase rear was closed by a spigoted rear cover secured by eight studs and nuts. The crankcase rear included the annular induction chamber and carried two magnetos, the oil pumps and the plain bearing at the crankshaft rear end. At the crankcase low point, between cylinder Nos. 5 and 6, a detachable external sump received oil draining from the engine interior . The sump was a two-piece aluminium alloy casting joined horizontally by six bolts and nuts. The upper half was formed with two hollow limbs that provided an attachment means and passed oil into the sump. Faced flanges at the limb upper ends mated with appropriate crankcase apertures, one in front and one at the bottom. At the sump rear a faced aperture with two studs attached the scavenger pipeline to the pump. This pipe extended through its flange across the sump exterior and was carried upwards at its inner end, maintain the oil level within, and preventing the entry of foreign matter from the sump floor. Another exterior pipe that drained the cam chamber entered the top half.

The crankcase rear face featured a spigot for attaching the engine to a bearer plate in the aircraft. The stub ends of the nine crankcase bolts were used to secure the engine to the plate. Crankcase interior surfaces were treated with a baked coating that reduced the risks of porosity and possible liberation of sand from the casting.

Crankshaft

Crankshaft, Oil Pumps, Auxiliary Drives

The one piece crankshaft was forged from chrome nickel steel and was borne in three roller bearings. The crank webs were fitted with counterweights. Each was located by a tongue that fitted into a corresponding groove in the extended web, and was secured by six bolts and nuts; the bolts were peened over to lock the nuts in place. The front crankshaft bearing was located in front of the thrust bearing; the intermediate bearing adjoined the front crank web front face, and the rear bearing the rear crank web rear face. Both were separated from the web itself by a thin radiused washer that took up the shaft radius. The rear and intermediate bearings bore the main loads and were of the crowded roller type. The front bearing was the caged roller type. The shaft front portion was threaded and machined with a splined taper to accommodate the airscrew hub. Immediately after the splines another thread was cut to receive the left-hand-threaded thrust nut that secured all the components assembled behind it to the shaft front. Three keyways cut through the thread provided for the locking washer that secured the thrust nut. The crankpin and shaft were hollow and oil passages were drilled down the webs to connect the hollow journal bores. Each end of the hollow crankpin was plugged by two coned cap drawn into their seatings by an internal bolt and nut. The shaft rear section bore ends were also closed by similar caps and securing fasteners. The shaft front bore was sealed by two threaded plugs; the one at the front was integral with the shaft, while that at the rear was provided with a hexagon nut and locking washer and was removable for cleaning purposes. A shallow oil way machined on the shaft for a short distance immediately in front of the front web supplied the cam gear. A duct drilled in the web in line with the oil way communicated with the main oil passage within and supplied it with oil. The shaft bore diameter varied; the first. portion to a point about midway along the serration was in the form of a decreasing cone; the remainder was of uniform diameter. More metal was removed from the shaft rear bore as it was not subjected to the bending and torsional forces of the shaft front.

The auxiliary drives were taken from the crankshaft rear end where two slots were cut there for engagement with their driving member, the magneto driving bevel. The slots engaged with keys machined in the driving bevel sleeve, which capped the shaft end. There was a clearance of 0.003" between the sleeve and shaft. The thrust bearing and cam gear were carried on the crankshaft front end between the intermediate and front main bearings. Each of these assemblies were mounted on a sleeve that was threaded onto the shaft, and the sleeves together with other shaft fittings serve as distance pieces (spacers) that were drawn up by the thrust nut.

Cylinders

Cylinder, Head and Barrel, Intake Valve

A steel barrel and an aluminium alloy poultice head comprised the cylinder; the head and barrel met in a face joint. The barrel portion was closed by the crown at the upper end save for four apertures cut out to form two inlet and two exhaust valve seats. The barrel was machined from a steel forging and its outer surface was provided with 30 fins of gradually increasing diameter towards the top, with the exception of the last seven, whose diameters were equal. The head was secured to the crown by eleven steel studs threaded into the crown and four removable bolt accessible via the valve ports. The studs varied in length, three being short and the remaining eight long; all were waisted and were of such length as to pass through the full head casting depth. The faced head and crown surfaces were carefully scraped at the joint in order to obtain full surface contact between the two, thus facilitating heat transfer from the cylinder to the head and making a gas-tight joint. Shallow recesses were cut in the inlet valve apertures of both; these accommodated phosphor bronze locating rings that were pressed into both parts to align the cylinder and head. Valve gas passages were formed in the aluminium head. The two inlet passages faced aft, while the exhaust passage axes faced forward at a 45° angle of to the crankshaft center line. The aperture flanges were faced and fitted with studs to receive the inlet and exhaust pipes; the exhaust outlets had four studs and the inlet two studs each. Three tapped bosses in the cylinder barrel provided for two sparking plugs and the gas starter check valve. The sparking plug bosses were situated at the engine back and the front, while the check valve boss was at the side. A small screwed boss adjacent to each sparking plug hole was attaching a bonding wire from the high tension lead metal braid. The cylinder base flange was of substantial proportions and was approximately square in shape with the corners machined to form a pair of lugs at each. A hole was drilled through each lug to receive a holding-down stud thus making eight studs for each cylinder. The cylinder spigot sides were cut away to clear the connecting rods; the clearance area in he six upper cylinders was greater than that in the lower ones owing to the master rod-auxiliary rod geometry. Bronze valve guides were housed in the aluminium head. Their outer surface was ground to a taper and was received in a corresponding boss reamed in the head. They were shrunk in position by heating the aluminium to a temperature of 250°C and inserting the cold guide. The exhaust valve guide bore was larger and longer than that of the inlet. A collar on the guide about 0.100" above the boss served as a seat for the inner valve spring but also controlling the guide's insertion depth by inserting a feeler gauge between it and the boss. Two of the studs that secured the head to the barrel also attached the rocker bracket on which were mounted the valve rocker arms. A pair of upwardly extended lugs cast at the head front received the bracket between them, and steadied it against 1atera movement.

Pistons

Piston		Oil Ring	Gudgeon Pin

The slipper-type aluminium alloy pistons had concave crowns and were each fitted with two cast-iron compression rings and one cast-iron oil scraper ring. The oil scraper ring, which was wider than the other two, was grooved to form a U-section, and holes were drilled at the U-section bottom to conduct oil to drain holes drilled through the piston wall at the ring back. A recess cut below the oil scraper groove collected oil; holes drilled in this recess drained oil from below the scraper. A hollow floating gudgeon pin of air-hardened steel featured a shoulder at one end that limited its axial travel. At its other end a stud projected from the partially closed pin bore and a plate was secured to the stud by a nut and split pin. A slot machined across the top of the gudgeon pin received this plate, securing the pin axially, but leaving it free to rotate. A short lug cast within the piston on either side between the two webs that carried the gudgeon pin bosses prevented the piston skirt from falling against the connecting rod when the cylinder was removed. The lugs was formed so that the rod encountered them before the skirt. Two oil holes in each gudgeon pin boss, and two others in the connecting rod small end provided gudgeon pin lubrication.

Connecting Rods

Connecting Rods		Wrist Pin, Master Rod Big End

The connecting rod assembly consisted of a master and eight auxiliary rods that were carried between two flanges formed on the master rod big end. An unusual feature of the assembly was the plain split phosphor bronze white-metalled master rod big-end bearing. The H-section rod shank cross section area gradually reduced from the large to the small end. A phosphor bronze gudgeon pin hearing was shrunk into position and was secured by two threaded brass dowels. The two flanges formed around the master rod big end were machined with eight holes that accommodated the auxiliary rod wrist pins. The holes were spaced apart at slightly unequal intervals to compensate for unequal articulated rod strokes, which, owing to the angular movement imparted to the big end by the master rod obliquity, rendered the stroke unequal in any two consecutive auxiliary cylinders. The four bolts that clamped the two master rod bearing portions together were housed in deep lugs formed on the flange outer periphery. The wrist pins were hollow except that an internal collar was left in the pin at about one-third of the length from one end; This served to anchor a securing bolt by which the pin was held in position. One end of the pin was ground with a short taper while the remainder was of uniform diameter, including the whole surface upon which the articulated rod was borne. Suitably tapered holes in the flange received the wrist pin tapered ends. A steel cup closed this pin end, and the cup, which was a larger diameter than the pin, bore upon the master rod flange surface. The securing bolt was inserted from the wrist pin opposite end and projected through the cup. A nut screwed on to the bolt and locked by a split pin held each pin in place. The two upper wrist pins closest to the master rod joint were smaller than the remainder in order to maintain the necessary material cross section in the big end between the clamping bolt and the wrist pin. These two pins had no taper on their surface but were a parallel fit in both master rod flanges; they were located axially by a shoulder at one end and a locking plate secured by a nut and stud at the other.

The auxiliary rods were of uniform section throughout their length. Both ends were fitted with phosphor bronze bushes and secured by the same method as that employed for the master rod small end. Two holes were drilled in each end for lubrication.

Two shallow recesses formed around the big end bearing inner surface were cut co-laterally with the flanges that carried the wrist pins; these relieved the pressure exerted on the pin by the bearing surface beneath the flanges. When the bearing warmed up under running conditions the flanges acted to locally cool the master rod in their vicinity. The portions of the master rod hearing thus cooled expand less than the remainder, and the recesses prevented contact with the crankpin, which expanded uniformly throughout its length. A small arrow engraved on the big end front face indicated the direction of rotation, and guarded against incorrect assembly of the rod on the crankpin. Immediately below the arrow a lightening hole bored in the rod provided metal that could be removed to balancing each rod.

Cam Gear

Cam Gear and Drive

Gear Drive	Fig. 20	Fig. 22	Gear Drive

The cam gear was located immediately in front of the crank chamber and was housed in a circular casing. Tappets were arranged radially around the cam gear casing with the inlet tappets in the front plane and the exhaust tappets in the rear, parallel plane. Each set was operated by a cam disc, which took the form of a circular plate with four lobes operating the inlet valves and four operating the exhaust valves. The cam disk rotated opposite the crankshaft at 1/8 crankshaft speed. A compound epicyclic gear train established the appropriate speed reduction and reversed the rotation direction. The whole assembly was mounted on the crankshaft with the exception of a large fixed internal gear, which was bolted to the crankcase front wall.

The cam assembly comprised an eccentric keyed to the crankshaft upon which was freely mounted a compound pinion with a 68-tooth external gear at one end and a 68-tooth internal gear at the other. This pinion's external gear, moved by the eccentric, rolled around the 72-tooth internal gear affixed to the crankcase. Its internal gear teeth were also in contact with a 64-tooth external gear on the cam sleeve, which was mounted concentrically on the crankshaft. This is shown in Figure 20, where the two cam gear trains (i and ii) are shown separately in plan view, along with the sectioned assembly (iii). Figure 20(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. B's speed increase in relation to A will be 72/68 in the anticlockwise direction. Simultaneously, as seen in Figure 20(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.

Figure 22 shows that the cam sleeve is a disc with a deep rim formed around its periphery and a bearing housing at its center. Two rings with four cams each were machined around the rim; the fore ring operated tappets and the aft ring the exhaust tappets . A 64-tooth gear internal gear wheel was located in the annular space between the rim and bearing housing. The gear wheel was pressed onto a spigot and secured to the web by eight bolts fitted in reamed holes, fixing the members rigidly together. The web was drilled for lightness. The cam sleeve was borne on an air-hardened steel sleeve threaded onto the crankshaft and abutting the main intermediate crankshaft bearing. The cam sleeve was lined with two phosphor bronze bushes pressed into either side of the sleeve center . The internal bush faces were suitably grooved to conduct oil over the whole surface, and holes drilled in the air hardened sleeve provided for a supply to the bearing from an oil way cut on the crankshaft. The right-handed groove in one bearing and left-handed groove in the other conducted oil away from the center recess. The length of the whole bearing was slightly less than its journal, which allowed for the next member threaded onto the crankshaft to be drawn up hard against the journal while leaving a working clearance for the cam sleeve.

Eccentric B, which was internally milled for lightness, was keyed to the crankshaft by a feather key. Its outer surface carried the compound pinion, which ran on a floating cast-iron bush. The bush was drilled for lubrication and an oil way from the crankshaft oil way supplied oil via a hole cut along the eccentric. A lip at the eccentric aft end helped locate the pinion axially, while a ground steel eccentric ring at the eccentric front captured the pinion at that end; a locating screw threaded into the eccentric prevented eccentric ring rotation. The pinion, which was slightly shorter than the eccentric length, was free to rotate about the eccentric. The eccentric ring, which was available in several thicknesses, formed a spacer that accounted for manufacturing tolerance build-up when the engine was assembled. Thus the intermediate main bearing inner race, oil thrower ring, crankshaft sleeve, eccentric, eccentric ring, front main bearing inner race, crankshaft nut locking washer and crankshaft nut were drawn up onto the crankshaft by the left-hand threaded crankshaft nut.

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

The fixed internal gear was a steel plate with an annulus at it center and was secured on the studs that secured the front cover and by a ring of studs on the inner tappet ring. Spigots on both sides near its outer periphery located it between the crankcase front surface and front cover rear surface.

Valve Gear

Clearance Compensation	Rockers, Diaassembled, Assembled	Fulcrum Pin Lubrication	Push Rods	Tappets

Jupiter valve gear included a device that compensated for excessive tappet clearance caused by cylinder thermal expansion. Each cylinder had a bracket that pivoted at the cylinder head center and was linked at the opposite end by a tie rod to the crankcase. A boss accommodated the rocker arm fulcrum pins, which were placed midway along the bracket. The bracket was steadied from lateral movement by two lugs cast on the cylinder head. As the cylinder expanded the tie rod remained constant in length, causing the fulcrum pin to be raised approximately only half the distance that the bracket pivoted end traveled. The fulcrum pin dropped relative to the cylinder head, taking up the excessive clearance that would otherwise he caused by the tappet rods remaining at constant length while the cylinder expanded.

The rocker arms for the two valve pairs were both pivoted upon a common fulcrum pin that was located by a grub screw passing through the bracket and entering a hole in the pin. The screw was secured by a locking wire. The boss on the bracket that accommodated these was forked; a single long rocker arm, formed with two branches at its operating end, lay within the fork and operated the two inlet valves. The two shorter arms lay outside the bracket and operated the exhaust valves. The two shorter arms were joined by a case-hardened cross bolt where they engaged the push rod. The bolt was turned down at each end to receive the arms, the center forming a spacer against which they were drawn up by the nuts. The nuts were locked in position by peening over the bolt ends. Both rocker arms were bushed with phosphor bronze. Short case hardened screws at the arms' operating ends provided valve clearance adjustment. At the actuated end the inlet rocker arm had a hemispherical socket that received a small hardened steel platform against that the hardened hemispherical push rod tip operated. A case-hardened steel stirrup sweated into the exhaust push rod end bore upon the case-hardened exhaust rocker arm cross bolt mentioned above. The hollow fulcrum pin was sealed at each end and carried a supply of oil for rocker lubrication. One end was closed by the head of a hollow duralumin bolt, and the other by a cap that was drawn on to the shank of the bolt by a check valve connecter by which an oil gun could be attached for filling the pin. Since the pins were at various angles to horizontal, the duralumin bolt was formed with two collars on its shank; these divided the hollow fulcrum pin into three compartments. Each compartment was fed by a hole in the bolt's hollow shank and supplied one of the three rocker bushes through four holes drilled in the hollow fulcrum pin. When the fulcrum pin was assembled the oil gun connecter was placed at the pin's upper end.

The push rods were ball-ended to fit into cups in the case hardened steel tappets. External springs were fitted to each push rod adjacent to the cam chamber; they were captured between a shoulder on the push rod and a bracket on the tie rod. These springs helped relieve the valve springs of the push rod and tappet inertia load. The case-hardened steel tappets worked in phosphor bronze guides 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 upon a hardened pin, which was a working fit in both roller and tappet and was located endways by the tappet guide. The guide inner end was similarly slotted to receive the roller and keep the tappet from revolving. The tappet guides were formed with a flange at the outer ends and each was secured to the crankcase by two studs. Threaded onto the studs after the guide was a steel plate, which in the five upper cylinders was drilled with a hole just large enough to allow the push rod to pass through. The plate was intended to prevent dirt or foreign matter from settling in the tappet cup. In the four lower cylinders this was unnecessary, as gravity would prevent any such contamination; the plate was therefore cut away at the center and was only used as a packing washer. The tappet guides of the four lower cylinders differed from the remainder in that they were provided with an oil baffle to prevent oil from splashing directly onto the tappets and working out under the influence of gravity. A trap was formed around the guide outer end, and oil collecting there was drained back to the sump by radially drilled holes in the guide wall. In the foremost tappets these holes were masked by the walls of the bosses of the crankcase in which they were housed, and the drain holes of these tappets were therefore interconnected with the rearmost tappets by two holes drilled in the metal of the boss that separated them, thus enabling oil from the fore tappet guides to drain into the aft ones and thence to the sump.

Valves

Valves, Inlet, Exhaust

Jupiter inlet valves had larger-diameter bell-shaped heads and smaller stems than the exhaust valves. The exhaust valves were made of a more heat-resistant material and their heads were slightly concave . To resist wear from the rockers, a case-hardened steel cup was fitted to the valve stem end of all the valves. Two concentric springs were fitted to each valve, and the valve spring washers were anchored to the stems by means of split collars. Three grooves were machined in the stem and shoulders in the split collar engaged with these. The outer surface of the collar was tapered and was received in a suitable seating in the valve spring washer. To avoid the harmful effects of the valve spring seating on the soft aluminium cylinder head, protective steel washers were interposed between the spring and the head.

Airscrew Hub

Airscrew Hub, with Huck Starter Claw

The airscrew hub rear flange and sleeve were turned from a steel drop forging. The sleeve's tapered internal bore was machined with serrations that corresponded with those on the tapered crankshaft. At the forward end a conical split phosphor bronze collet mounted on the shaft fitted into a corresponding cone in the sleeve and was retained by a serrated screw cap that screwed onto the crankshaft end. The purchase obtained by this cap drew the sleeve hard up on the taper splines and locked it firmly on the shaft. The serrated cap was locked in position by a plate that was threaded onto a stud projecting from its center. The plate engaged serrations on the nut and also with others cut around the crankshaft end, thus locking it to the crankshaft. The loose hub front flange engaged with parallel serrations cut on the sleeve's outer surface and was free to move down the sleeve to the depth required to clamp the airscrew. Ten steel bolts passed through the flanges and airscrew the boss, and were secured by nuts and tab washers on the front face. To prevent the bolts from turning, a rib was formed around the rear flange rear face and engaged the bolt head flats. The sleeve outer surface was turned with a thread for some distance from the front to accommodate the large clamping nut. A tab washer, which was bolted to the front flange, locked this in position.

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