Continental XO-1430 Development
Part 11: 1938
by Kimble D. McCutcheon
Published 17 Oct 2025; Revised 18 Oct 2025

20 – 22 Jan 1938. Continental's Chief Engineer Norman N. Tilley and Design Engineer James W. Kinnucan met at MatCmd with Power Plant Laboratory Chief Maj. Edward R. Page, civilian engineers Opie Chenoweth and Ford L. Prescott, and with Aircraft Laboratory civilian engineer D.A. Dickey. Tilley and Kinnucan had brought with them failed XO-1430-1 reduction gear parts for examination by MatCmd personnel. The reduction gear pinion appeared to have been operated in a misaligned condition, permitting the gears to be overloaded at one gear-tooth end. This loosened the spline fit between the supercharger drive gear hub and the reduction gear pinion shaft (the reduction gear pinion bearing was on the supercharger drive gear hub outside). Two fixes were discussed and the most logical could be incorporated in the reduction gears then being fabricated. In this design, the supercharger drive gear was to be integral with the reduction gear pinion shaft and the reduction gear pinion was to be splined onto this shaft with a slight shrink fit; with this scheme misalignment would be virtually impossible unless a reduction gear hub bushing failed, but they were lightly loaded. Upon disassembly, it had been found that several reduction gear pinion teeth had broken and three or more pieces had gone through the second-stage gear pair. The second stage gears were not injured, but the propeller shaft flange was cracked in three places, presumably by deflection caused by a gear piece passing through the gears. The gears were marked where the pieces passed through, but no damage was detectable. Repair would include a new propeller shaft and a new first-stage gear set; fortunately the parts on order were at a stage such that the improved design could be incorporated.

The No. 4 main bearing web and bearing boss had cracked almost all the way to the cylinder deck. This was first blamed on some bearing load phenomenon not associated with the reduction gear breakage. However, a missing herring-bone tooth fragment had imposed a very heavy thrust load on the crankshaft, which was transmitted to the thrust bearing on the No. 4 main crankshaft journal. The spare crankcase on order was not yet being machined, so the No. 4 bearing boss thickness could be increased by 0.063". No other crankcase anomalies were detected, so no further crankcase design changes were contemplated. It was later found that this scheme could not be assembled. An alternate design that centered the supercharger drive gear using two ground lands about 0.375" wide on either side of the splined portion was to be presented to MatCmd.

The Aircraft Branch Propeller Unit discussed propeller hydraulic control oil supply redesign. A satisfactory arrangement was identified and Continental was to produce a layout and present it to MatCmd. The participants discussed a nose section sump whose oil level would be maintained by a jet from the engine's main oil; all thought a submerged propeller control oil pump would be very desirable; this would leave the propeller control a separate matter and would divorce the propeller operation from the engine oiling system. Kinnucan promised layouts.

Kinnucan stated that Continental was experiencing considerable difficulty in properly locating the reduction gear housing on the crankcase forward flange. A slight misalignment caused bearing difficulty in the front accessory drive forward bearings. A front accessory drive redesign incorporating the forward bearings in a separate bearing bracket bolted to bosses on the forward main bearing diaphragm was proposed. However, this bracket required the reduction gear housing rear diaphragm be redesigned and introduced offsets in the diaphragm; it also would not have the same rigidity and strength. It was therefore proposed that the reduction gear housing be dowelled to the crankcase front using five good-sized dowels around the flange. Kinnucan agreed to prepare layouts for inspection. [USNARA RG342 P031118. 10 Jan 1938 Memorandum Report E-57-285-73, Conference with Representatives of Continental Motors Corporation, January 20, 21 and 22, 1938, Regarding Development Test of XO-1430-1 Engines.]

Fast's Copuling (RegalRexnord)

8 – 9 Feb 1938. Kinnucan and Prescott met at Wright Field to discuss the recent XO-1430-1 reduction gear failure and resultant center main bearing thrust face damage. During engine operation Continental had observed that the Fast's coupling that attached the engine to the dynamometer had a tendency to float fore and aft under steady load, which was theorized to place considerable end thrust on the engine. The XO-1430-1 propeller shaft single-row thrust bearing had about 0.020" end play under thrust loads; this end play was transmitted through the reduction gears, quill shaft, to the crankshaft and finally to the No. 4 main bearing thrust face. Since the reduction gear herring-bone angle was about 30°, the normal tooth load imposed by this force would be double the end thrust. This force would be added to the normal driving force imposed on the gears and was thought sufficient to account for the initial gear failure. A small misalignment, such as was undoubtedly present because of the supercharger drive gear loose spline fit, would accentuate the gear teeth overloading. This was the explanation that Kinnucan and Prescott concocted to explain the recent failures.

Continental had prepared a new layout featuring two ground 0.375" lands on either side of the splines, resulting in a light press fit on assembly and presumably eliminating any looseness, which should eliminate future reduction gear misalignment. Kinnucan proposed to study the spline and gear tooth fits on the quill shaft in order to determine if the quill could slide under load. He suggested a test rig that could subject the entire assembly to normal driving torque and then measure the force required to slide the quill shaft under load. The results would be incorporated in the new reduction gears and quill shaft being fabricated.

Kinnucan pointed out that the incorporation of propeller shaft splines to drive the propeller governor drive gear resulted in a 50% loss of propeller shaft thrust area against its thrust bearing. It was therefore proposed that propeller governor drive gear be driven by a key instead of splines since the load transmitted to this gear was small and the key would be ample for driving this gear. This would allow the entire propeller shaft thrust surface to be retained. A revised propeller control and reduction gear layout was to be mailed to MatCmd as soon as available, and the engine was promised to be available for testing within 4 – 5 weeks from this conference date.

Kinnucan brought two rough-machined crankcase castings for X-ray inspection. Neither personal nor equipment was then available for this purpose. The castings were then shipped to ALCOA, Cleveland Branch for X-ray and the results were forwarded to MatCmd; the crankcases immediately entered the machine shop. Kinnucan said that machining these castings was to be expedited. [USNARA RG342 P031112. 12 Feb 1938 Memorandum Report E-57-285-74, Conference with Representative of Continental Motors Corporation on XO-1430-1 Engine.]

11 Mar 1938. Kinnucan met Prescott to discuss, among other things, the necessity of a 0.005" limit on Dwg #X38B2769 spring rings. MatCmd agreed that this limit could not be held due to the spring ring flexible nature and there was no necessity for holding the close tolerance called for in the drawing.

Kinnucan reported that a test ring had been built to determine the sliding force necessary to move the quill shaft in the XO-1430-1. It was found to be over 5,000 lb for rated engine torque, making it evident this was responsible for the reduction gear and crankcase failure during the last test run. Dwg #500705 was prepared showing a modification whereby the crankcase and reduction gears would be relieved of differential expansion stresses. The only objection to this arrangement was the necessity of removing a cylinder to install or remove a retaining ring nut. This was not thought to be serious although further study might uncover a method to overcome this objection.

The desirability of having an XO-1430-1 alternative mount scheme was discussed. The present mount plate arrangement was not satisfactory and it was desirable to have another mounting method ready for adoption. Kinnucan said that a 4-point automotive-type mounting scheme would be drawn up to relieve the cylinders of all mounting loads.

Kinnucan also stated that a layout would be prepared to replace the current herringbone reduction gears with a spur-gear pair; this would completely overcome the difficulty experienced with differential expansion and relieve the crankcase, reduction gears and crankshaft of stresses due to the sliding splined quill shaft under load. A ball bearing manufacturer was preparing an estimate for a ball bearing spline intended to reduce the force required to slide the spline. However, there was concern that the balls might brinnell the splines and aggravate the condition. Deflection due to torque would also preferentially load the first few balls with the remainder carrying no load. [USNARA RG342 P030318. 18 Mar 1938 Memorandum Report E-57-285-75, Conference on XO-1430-1 Engine.]

12 Apr 1938. Prescott and Lt J.W. Sessums visited Continental where Kinnucan displayed some parts for the MatCmd special test engine whose fabrication had been made difficult by the Air Corps drawings. One drawing covered two different crimped spring packing ring sizes in which a minimum deflecting force was specified. Large rings made to the drawing just met the minimum specified deflecting force, while smaller rings required approximately double the specified deflecting force. MatCmd replied that these rings were intended to pack a Prestone joint in conjunction with a synthetic rubber ring; the deflecting force could be adjusted by changing the ring number and deflection, and that the rings already fabricated would be satisfactory for this purpose. There was also a piston ring set for 3.625" cylinder bore in which the load required to close the gap was specified. However, one was a beveled oil ring, which could not have the same ring pressure as a compression ring. The compression ring pressure fell within the drawing limit, but the beveled oil ring pressure was about one pound lower. Again, MatCmd thought the rings would be satisfactory and suggested a letter identifying the discrepancies from the drawing and requesting acceptance by deviation.

An XO-1430-1 crankshaft exhibiting a number of discrepancies was examined; none of the discrepancies appeared serious. Two previous crankshafts had not exhibited these discrepancies. MatCmd thought carelessness in finish machining was responsible for the numerous errors. MatCmd also did not think any of the discrepancies were serious and suggested requesting a deviation; if the crankshaft was accepted, MatCmd would file a letter of protest and summarily reject future items with discrepancies of this magnitude and nature.

Revised engine mount drawings depicted a transverse mounting plate clamped between the reduction gear housing and crankcase, and another transverse plate clamped between the crankcase and accessory housing. This would comprise part of a flexible 4-point mounting system, with each of the four points in Lord-type mounts. This mounting method would completely relieve the cylinder barrels of bending and torsion engine mounting loads. Continental planned to study this scheme further and ultimately submit drawings to MatCmd for official release. An alternative mounting scheme in which the four mount points would be attached to the ends of two mount plates that were clamped between the cylinders and cam boxes. MatCmd stated that this method would result in the forward cylinder pair transmitting nearly the entire propeller torque load; MatCmd thought the first scheme preferable.

Drawings showing the incorporation of a two-way hydraulic propeller control were examined. MatCmd suggested a drawing showing a separate propeller governor oil pump that would be submerged in a small oil sump in the engine nose that was filled by the engine oiling system. Using this modified propeller governor scheme the high demand on the engine oiling system as then required by the propeller governor specification, would be eliminated and oil would always be ready for actuation of a propeller control. MatCmd suggested that this revised propeller control scheme be brought before interested MatCmd personnel for discussion.

The XO-1430-1 that had just been rebuilt was being brought up to power preparatory to starting the 50-hr development test. Engine operation was noticeably smoother than at any previous time. This was partially attributed to the efforts taken to prevent undue end thrust between the crankshaft, crankcase and reduction gears; these factors, which were caused by thermal expansion, were blamed for previous reduction gear and crankcase failures. [USNARA RG342 P031105. 14 Apr 1938 Memorandum Report E-57-285-77, Visit of Lt. Sessums and Mr. Prescott of the Division to Continental Motors Corp. on April 12, 1938, Regarding XO-1430-1 Engine Development Test.]

4 May 1938. Prescott traveled to Continental and met with Kinnucan, who stated that several inner propeller shaft bushing had failed without explanation. He also said that the front accessory drive bearings always appeared scratched after engine operation. Deflection tests were to be made on the propeller shaft and crankshaft reduction gear pinion to determine if excessive deflection was causing the bearing failures. The older propeller shaft design deflected about 0.006" with the 12,000 lb rated load applied to the testing machine. The circular propeller shaft section under these conditions became elliptical, with the major axis approximately 0.006" greater than the original diameter. The latest propeller shaft design was tested, and it went out-of-round approximately 0.008" with an 8,000 lb load and 0.012" with a 12,000 lb load. The early shaft design had a magnesium plug that provided some section stiffening. It was thought that this deflection accounted for all the difficulty experienced thus far. The proposed remedy was to install a steel stiffener immediately adjacent to the bearing shell, which would prevent propeller shaft deformation under load. The reduction gear pinion shaft was also tested and found to go out-of-round by 0.003" at a 4,500 lb rated load; a stiffener was also designed for this part. It had previously been necessary to supply about 0.006" clearance to the outer propeller shaft bearing, which apparently allowed the shaft to deflect under load without crimping the bearing. However, clearance of the unloaded side must have been increasing from 0.006" to 0.012" or more, thus permitting excessive oil flow through this bearing; the new design would deflect less than 0.001" under load. Continental was hopeful these measures would completely overcome the reduction gear bearing difficulties.

The front accessory drive bearings were examined to determine why the wear and scratching was occurring. Several of these bearings had an oil hole at the bearing center, but the blind holes into which these were pressed had no provision to drain the oil that flowed in, meaning that there was no washing action over half of the bearing surface. To address this a means of introducing the oil very near the bearing blind end was proposed, which would allow the oil to wash through the bearing, leaving it clean. A better way of introducing oil to the eccentric bushing of an idler shaft was evolved and led to the design of a new eccentric bushing with more bearing area between the bushing and crankcase and also improving the eccentric bushing lubrication. Kinnucan thought that these three measures would overcome the bearing failures and enabled completing the 50-hr development test. An XO-1430-1 with modified lubrication and shaft stiffeners was promised by 5 or 6 May 1938.

One cylinder barrel made from SAE 4130 steel had given excellent service. The previous SAE 4130 steel difficulties were thought to be due to cold grinding rather than the material itself. None of the hot-ground cylinders had exhibited the excessive piston ring and cylinder barrel wear, and the SAE 4130 strength and fatigue properties were superior to the SAE 1050 steel cylinder barrels. Oil flow through the engine was still high although oil consumption was satisfactory. Kinnucan thought that the current changes would result in a significant oil flow decrease in the reduction gear assembly. No effort had been made to reduce the rear accessory housing oil flow because no difficulty had been encountered with this assembly; Continental felt that no changes should be made until after the development test. Thereafter, the rear accessory oil flow would be metered to reduce its flow to a reasonable value. [USNARA RG342 P031099. 6 May 1938 Memorandum Report E-57-285-78, Conference with Representatives of Continental Motors Corporation on XO-1430-1 Engines.]

13 Jun 1938. Prescott met with Kinnucan at Continental to examine the unsatisfactory reduction gear condition after only five hours running. The gears were pitted along the tooth pitch line and the tooth wear on the addendum and root was excessive. Mr. Kitchen of the gear manufacturer, Farrel-Birmingham Company, Buffalo, New York, was present and stated that some form of deflection was occurring under load conditions and was causing the tooth pitting and wear. He suggested a test setup using the old reduction gear housing and gear train, which could be loaded at rated torque and the deflection measured using dial indicators and rods passing through the gear case side and contacting the gears. Continental immediately started to set up the equipment to conduct this test.

The gear appearance was unsettling considering the short time they had been in service. Because of their herringbone shape it was impossible to grind the gear teeth after machining. A smooth finish was achieved by first using a cast-iron lapping gear followed by lapping the gear pair that would operate as a set. The amount of material removed by lapping depended on how much sliding contact the engaged gears made; tooth areas above and below the pitch line lapped much faster than areas at the pitch line. This lapping operation modified the tooth form so that the finished gears did not have true involute tooth surfaces. This resulted in severely reducing the effective number of teeth in contact, giving a large increase in material unit loading at the point of contact. Since there was no other way to finish herringbone gears, Prescott feared that they might have to be abandoned for such highly-loaded applications and suggested that Continental begin preliminary layout work on a reduction gear using ground spur gearing. He also suggested laying out an epicyclic gear train such as those used in large radial engines.

Just prior to its last shutdown the No. 1 connecting rod bearing failed in overspeed because the bolt used to tie the quill shaft and crankshaft together broke; this was the second such failure and another scheme was needed for this connection. Prescott and Kinnucan thought a satisfactory solution was to move the crankshaft locating (thrust) bearing from the No. 4 main journal to No. 7; Kinnucan said that a sketch covering this change would be prepared and that the crankshaft would be sent to the Ohio Crankshaft Company to have the No. 1 crankpin reground. The replacement connecting rod bearing would feature a central groove as the failed bearing lacked the groove. Three blade rods had an approximate 0.006" clearance so it was decided to install enough new bearings to reduce the excessive blade rod clearances to the nominal value that appeared to have worked well on the other crankpin bearings. One other un-grooved bearing was on the No. 4 crankpin. It was decided to further test this bearing to determine if its construction was problematical. The disintegrated bearing material was ground up so fine that much of it went through the sump and caused several bushings to be scratched throughout the engine; despite the scratches, all but two or three accessory housing bushings were still serviceable. A final decision on the reduction gear fix was awaiting deflection test completion. Kinnucan planned to return to Detroit around 15 June, examine the test results, and recommend a path forward.

Kinnucan said that spark plugs with bottom-seating gaskets located at the spark plug bushing inner ends had been procured and would be tested when the 50-hr development test continued; he thought this spark plug arrangement would be superior. [USNARA RG342 P031100. 17 Jun 1938 Memorandum Report E-57-285-79, Conference at Continental Motors Corporation on XO-1430-1 Engine Test.]

8 Jul 1938. Kinnucan met Prescott at MatCmd and brought calculations made for XO-1430-1 spur reduction gearing to replace the herringbone gearing then in use. He had determined that operating conditions for hardened and ground spur gearing were very good ad that spur gearing could be fabricated and installed in about three weeks. Prescott and Kinnucan discussed stiffness and flexibility of various reduction gear components, including methods that the inherent ring gear flexibility could be used to ease the internal gear teeth stress; the present herringbone gear deflection under load concentrated the loading upon small gear teeth portions, causing breakdown, particularly at the pitch line near the herringbone teeth apexes.

Kinnucan inquired as to the suggestion offered by MatCmd of modifying the supercharger scroll to provide additional resistance to backfiring. If the supercharger scroll outer wall flat portions of were made slightly convex, its strength would increase without a corresponding housing weight increase.

Kinnucan stated that Continental had employed additional personnel in order to expedite XO-1430-1 completion. The engine was again in operation using the present reduction gearing and the engine condition after run-in was satisfactory. He said that upon reassembly the engine would immediately start endurance running toward the 50-hr development test. [USNARA RG342 P031095. 12 Jul 1938 Memorandum Report E-57-285-81, Conference with Representative of Continental Motors Corporation on XO-1430-1 Engine.]

1 and 5 Jul 1938. Continental's Tilley and F. Gould met with MatCmd's Maj Page, Lt Sessums, Lt Robey, Chenoweth, Clawson and Prescott at MatCmd. Maj Page stated that the conference purpose was to outline the XO-1430-1 development policy; MatCmd thought that expending approximately $241,000.00 over a five-year period should have resulted in an engine within two years after the promised 26 Aug 1936 delivery date. Tilley stated that in justice to Continental many changes were necessitated by the failures experienced in testing and that it was his feeling that the major changes necessary for project completion had already been made. He believed that it was essential to change the reduction gearing from the herringbone type to plain spur gearing that could be accurately ground after hardening. He also recommended, in view of the unsatisfactory spark plug service life, that a new type of spark plug insert incorporating a bottom seal gasket be used.

Maj Page said that MatCmd's feared that 1,000 hp class engines would be underpowered by the time the 50-hr development test and 150-hr type test were passed. Tilley said tat a larger engine could be built around the same major parts as used in the XO-1430-1 and suggested that continued development could not be considered time lost on the development of larger engines. He felt that the step from a 12-cylinder design to a 24- or 36-cylinder design was a comparatively minor step once the 12-cylinder design was completely developed. MatCmd pointed out the desirability of retaining another potential aircraft engine source, and that it was not desired to abandon the XO-1430-1 project, but rather accelerate its development in order to ultimately produce a higher-power version using the parts then under development for the 12-cylinder engine. Mr. Gould stated that he had been commissioned by Continental to draw up a mutually-acceptable development program. He had prepared a schedule showing approximately $14,500.00 due upon completion of the 50-hr development test under the present contract, and an additional $16,000.00 to carry the project through completion of the 50-hr development test. He opined that it would be possible to complete the project in approximately two months. However, he wished to base his program on approximately four months dating from 1 Jul 1938. Gould was directed to confer with Continental officials and submit a letter proposal for formal Air Corps acceptance. Gould agreed to do this immediately upon his return to Detroit and promised the proposal would be in Air Corps hands within a few days. [USNARA RG342 P031097. 12 Jul 1938 Memorandum Report E-57-285-80, Conference with Representatives of Continental Motors Corporation on XO-1430-1 Engines.]

14 Jul 1938. Prescott met with Kinnucan and Massachusetts Institute of Technology Professor Earle Buckingham to discuss the XO-1430-1 propeller reduction gear. Kinnucan showed sketches of the revised reduction gear incorporating ground spur gears in place of the herringbone gear set used in the original design. He also presented stress and wear calculations, which Buckingham thought entirely satisfactory. Buckingham also suggested that the second reduction gear stage be made with as large a reduction as possible in order to reduce gear loading. A gear set was evolved in which the exact 2:1 ratio was obtained with satisfactory stress and wear factors.

The herringbone gears had forced various gears in the train to assume definite position, located by the engine thrust bearing. No provisions existed to resolve the spur-gear thrust loads. However, a simple and effective means to locate the crankshaft and secondary shaft gears was evolved. Buckingham stated that perfect herringbone gears, in theory, showed strength and wear advantages over equivalent spur gearing. However, the application of these gears using case-hardening material had to be finished by lapping. This was objectionable because no lapping took place at the pitch line, where pure rolling took place between the gears, and the lapping was proportional to the distance from the pitch line. Consequently, herringbone gears, when lapped, did not have a true involute profile and were not as effective as spur gears ground to the correct involute form after hardening. Kinnucan promised to provide, for MatCmd's approval, a revised gear set layout on 19 Jul 1938, together with detail drawings of the various components. Gear blanks had already been ordered, fabrication would proceed immediately, and delivery was expected in about three weeks.

While the engine was operating during the last test period a slight oil pressure drop was observed along with the loss of about 60 lb loading; the engine was shut down for examination that revealed a front accessory drive idler gear shaft had worn excessively and broken off, leaving the gear supported with only one bearing. No adequate explanation was found, although the gear shaft's journal and bearing had been scratched by bearing metal from a previous rod bearing failure. Ultimately, it was determined that this failure was insufficient to condemn the current accessory drive scheme and that new bushings would be fitted to a new gear and engine operation resumed during the week of 18 July using the herringbone gearing in order to get as much time as possible on other engine parts. [USNARA RG342 P031093. 18 Jun 1938 Memorandum Report E-57-285-82, Conference at Continental Motors Corporation Relative to the XO-1430-1 Engine.]

3 Sep 1938. Prescott met at MatCmd with Douglas Aircraft's W.B. Oswald and E.F. Burton to discuss Lycoming XO-1230 and Continental XO-1430-1 engines. Oswald stated that present aircraft requirements with constant-speed propellers would make it possible to eliminate or reduce the 25% overspeed requirement in the present Air Corps engine specifications. He stated that reducing this requirement to a value lower than 25% would probably result in more rapid completion and test of high-performance engines; this rapid development was desirable from the aircraft manufacturers' standpoint because studies concluded that the speed of two-engine bombers could be increased approximately 15% by installing these engine within the airplane wing.

Oswald said that the XO-1230 magneto projected from the engine's nose an objectionable amount for wing installation and suggested that it be moved from the top to one side in order to decrease the engine's height. He also objected to the coolant pump location for both the XO-1230 and XO-1430 as it increased the engine's height. Oswald was in favor of raising the emergency coolant-out temperature from 250°F to 300°F so a smaller coolant radiator could be used. Oswald also thought the XO-1230-1 generator facing straight back was objectionable and suggested mounting it using a right-angle adapter.

Oswald showed a design using an Allison V-1710 within the wing with its crankshaft perpendicular to the aircraft longitudinal axis and driving a propeller through a right-angle gearbox. MatCmd thought Allison in a good position to comment no the right-angle drive in view of its experience with airship right-angle drives. Oswald stated that there was a demand for flat engine with a 1,300 hp normal rating and 1,600 hp takeoff rating. He thought the engine should be kept thin and thus could not be double-deck horizontally-opposed engine, but rather a 16-cylinder engine similar to the XO-1230 or XO-1430. [USNARA RG342 P081127. 9 Sep 1938 Memorandum Report E-57-2051-1, Conference with Representatives of Douglas Aircraft, Inc. on XO-1230 and XO-1430 Engines.]

8 – 9 Sep 1938. Continental's Kinnucan and A.W. Wild met at MatCmd with Prescott and brought with them for examination the spur reduction gear set intended to replace the herringbone reduction gear. Both the first and second stage gears showed the effects of corner loading; this effect was more pronounced on the first-stage gears. The second stage gears were pitted near the corners nearest the propeller, and the gear appearance indicated that misalignment or deflection was partially responsible for the gear set's failure. Detailed examination concluded that the secondary shaft became misaligned in response to its reaction to first and second stage gear loads, and that this misalignment was not limited by the cold clearance of the plain bearings in which the secondary shaft ran, but also included the increased clearance when the gear case achieved operating temperature. The propeller shaft apparently suffered from the same difficulty although a plain bearing was used only at the shaft rear end. This tended to bring the propeller shaft and secondary shaft into a parallel operating position but caused the primary gear shaft to be misaligned in a direction opposite the secondary shaft. These misalignments also explained why the herringbone gears caused trouble, breaking their teeth down at the corners where the spur gears showed heavy loading. Study of previous designs incorporating two-stage reduction gears showed they all used roller bearings with no appreciable bearing clearance, which is why they had been successful in British engines even when their reduction gears were run at considerable overloads. It was concluded that shaft misalignment could be accurately determined and the spur gearing reground to compensate, allowing the spur gears to be used in continued testing that would clear the other non-reduction-gear engine parts. Kinnucan stated that the total maximum stress on the spur gear set was 260,000 psi, which was too high for available steels to have reasonable lifetimes. It would therefore be necessary to evolve a new reduction gear design incorporating steps to eliminate misalignment and reduce gear stresses to satisfactory values.

Continental proposed that the 50-hr development test be extended to the point that all engine parts except the reduction gear had cleared the test. Since Douglas Aircraft was interested in using the engine with a right-angle propeller drive. The preliminary design for this scheme had been accomplished and the Gleason Gear Company, maker of right-angle drives for the Akron and Macon airships, had been contacted about right-angle drives for Continental XO-1430 applications; this design would be submitted to MatCmd for approval in the near future. The proposed bevel drive construction was discussed at length and steps proposed whereby the spur-gear misalignment would be eliminated with the right-angle drive. The preliminary sketch showed this right-angle drive as an integral engine part; MatCmd suggested an alternative design be submitted that incorporated a complete separate right-angle drive connected to the engine by a drive shaft using external-internal gear-type universal joints at both ends.

Since Continental recognized an obligation to complete the spur gearset design, MatCmd ruled it satisfactory to carry the price of the spur gearing on a progress report with other replacement parts and adjust the new-design reduction gear price at a future date. Kinnucan said that the spur-gear design would proceed immediately so as to have this gear available for completion of the 50-hr development test on the entire engine, thereby satisfying the requirements of Contract W-535-AC-8131. This gear design would probably incorporate some form of needle or roller bearings to eliminate the misalignment encountered in the plain bearing type. [USNARA RG342 P031090. 14 Sep 1938 Memorandum Report E-57-285-83, XO-1430-1 Engine and Development Test.]

This proposal was discussed at length. MatCmd thought the changed direct-drive torsional characteristics might introduce engine part failures that would have successfully passed with properly designed reduction gear. Such testing would be outside Contract W-535-AC-8131, and the furnishing of replacement parts as called for under that contract would be extremely questionable, and that the objective of the contract would not be fulfilled by this procedure. Since only 15 hrs testing remained on the non-reduction gear parts, MatCmd had no objection to completing the 15 hrs so as to expedite the Douglas Aircraft work. However, during the discussion MatCmd learned that certain vital engine parts had operated for ONLY 15 hrs and it would be necessary to test the engine for another 35 hrs to for it to pass the 50-hr development test, less the reduction gear. PPL opined that all effort should be concentrated on furnishing the new reduction gear and completing the 50-hr development test, since the total testing time of the proposed plan would be 85 hrs provided not failures occurred. [USNARA RG342 P031087. 24 Sep 1938 Memorandum Report E-57-285-84, XO-1430-1 Engine and Development Test.]

Capt Carlson and Prescott examined a single-stage reduction gear preliminary design in which the gear offset could be to the right or left merely by swinging the gear case around on the engine. Capt Carlson and Prescott thought this reduction gear design was entirely unsuited to military aviation design; it was decided that further work along this line would be abandoned in favor of a concentric reduction gear. Upon returning to MatCmd Capt Carlson and Prescott learned that Continental's proposed program would not be approved until the Engineering Section Chief returned in about two weeks; the PPL position in Memorandum report E-57-285-84 remained unchanged, and temporarily setting aside Contract W-535-AC-8131 to take up the Douglas Aircraft project was premature. I appeared that both Continental and some MatCmd factions were jeopardizing the XO-1430-1 development project in order to use the engine in a Douglas Aircraft design. No testing other than authorized by the Contract was to be contemplated or prepared for prior to receipt of official test program release. [USNARA RG342 P031084. 26 Sep 1938 Memorandum Report E-57-285-86, XO-1430-1 Engine and Development Test.]

23 Sep 1938. During a telephone conversation between Kinnucan and Prescott, in answer to Continental's 22 Sep 1938 telegram, it was said that the de-clutching arrangement between water brake and dynamometer had never been used because it disengaged itself after running for a time. To prevent this, a groove was turned in the jaws and a two-piece ring bolted in place in this groove. It was stated that the proposed XO-1430-1 direct drive tests were to be dropped as they were of no interest to the Air Corps. Continental's new reduction gear should be designed and fabricated to continue operation under Contract W-535-AC-8131. Prescott requested a proposal on the new direct drive engine and right-angle gear box be submitted as a complete type test. [USNARA RG342 P031086. 28 Sep 1938 Memorandum Report E-57-285-85, Telephone Conversation with Mr. J. W. Kinnucan of Continental Motors Corporation on the XO-1430-1 Engine Development Test.]

30 Sep 1938. Kinnucan, Tilley and Wild traveled to Wright Field to meet with L.L. Lamb of the Gleason Gear Works and Maj. E.R. Page, and Prescott of MatCmd. Drawings showing various ways to improve the XO-1430-1 reduction gear design were examined. Among these were several spur gear sets including double reduction gears with needle bearings and single-stage offset reduction gearing, and a two-stage epicyclic gear. Unfortunately, all of these schemes presented difficulties that left some doubt as to the immediate success of the XO-1430-1 50-hr development test.

Zerol Gearset (Industrial Quick Search)

Lamb proposed the use of a "Zerol" bevel reduction gear, in which the teeth were cut on an arc, similar to a spiral bevel gear but with a 0° spiral angle. The Zerol gear tooth convex side radius of curvature was slightly less than that of the concave side so that these surfaces, when in contact, bore most heavily in the tooth face center and distributed their load over the teeth by deflecting under full load. This avoided corner loading failure and Gleason's manufacturing method permitted precision grinding so that the pinions and gears could be individually replaced. Zerol bevel gears provided a moderate tooth overlap, which improved smoothness, reduced vibration and minimized the thrust loads seen in spiral bevel gears. Lamb stated that the Douglas Aircraft requirements had been studied and suggested bevel gear drive pinion diameter be approximately 9" and that the gear face width not exceed 1/3 of the pitch cone length. Douglas Aircraft's requirement that the reduction gear depth not exceed 15", limited the reduction ratio; Gleason suggested a 8.85" pitch diameter pinion meshing with a 14" pitch diameter gear, the spiral bevel teeth with a 4.07" diametral pitch, and a 2.75" face width. With these gears the primary stage ratio was 1.58:1. In order to complete the reduction and bring the overall ratio to 3.5:1 as required by Douglas, a second stage with a 2.205:1 ratio consisting of a 12.65" pitch diameter fixed gear, 3.87" pitch diameter pinions, and a 10.5" pitch diameter driving gear. These used 4.66 diametral pitch with 1.75" face Zerol teeth. Lamb observed that if the driving and stationary gears had interchangeable hub dimensions, the entire gear set could be turned end-for-end and the reduction ratio reduced from 2.205:1 to 1.83:1, giving a two-stage overall ratio of either 3.5:1 or 2.9:1.

After this discussion it appeared desirable to make the bevel gear reduction set interchangeable between the Douglas Aircraft installation and the XO-1430-1, which would make the XO-1430-1 slightly oversize, but the gearing weight would not exceed the present reduction gear. Thus, the reduction gear development would proceed immediately in connection to the 50-hr development test and provide two optional gear ratios. Continental promised to expedite the reduction gear design and furnish MatCmd a type-test article cost estimate; approximately 4 weeks would be required from the time the MatCmd released the design. MatCmd emphasized the necessity for completing both project states at the earliest possible date so that XO-1430-1 development did not delay type testing the right-angle drive for Douglas Aircraft's wing installation. [USNARA RG342 P031079. 5 Oct 1938 Memorandum Report E-57-285-88, Conference with Representatives of Continental Motors Corporation and Gleason Works on the XO-1430-1 Engine.]

7 Oct 1938. Kinnucan, Tilley and Wild traveled to Wright Field and met with Maj Page, Chenoweth and Prescott. Wild presented an estimate for a direct-drive type test engine with a right-angle gear box giving a 3.5:1 reduction ratio. Items in doubt included the method of supplying the actuating oil to the hydromatic propeller installation. This matter was discussed with Propeller Unit personnel and it was decided that the proposed propeller actuation mechanism on the right-angle gear box rear side would be acceptable because it would be less difficult to make a sufficiently oil-tight small-diameter gland at this location. The disadvantages pointed out were the large seal diameter near the thrust bearing and the requirement that this seal be oil-tight against pressures as high as 600 psi. The matter of testing facilities for 1,000 hp at about 900 rpm was brought up and it was suggested that a built-in torquemeter be made integral part with the right-angle gear box; the proposal was revised and submitted to MatCmd. Right-angle gearbox details for the Douglas Aircraft installation were discussed. Continental was expediting this matter in order to proceed with XO-1430-1 development as well as furnishing a type-test engine for Douglas. Several minor items with the bevel planetary reduction gear were not thoroughly worked out and these were to be covered as quickly as possible and drawings submitted to MatCmd during the week of 10 Oct 1938. [USNARA RG342 P031077. 13 Oct 1938 Memorandum Report E-57-285-89, Conference with Representatives of Continental Motors Corporation on the XO-1430 Engine Development.]

14 Oct 1938. Kinnucan, Tilley and Wild met with Maj Page, Lt McCoy, Chenoweth and Prescott at MatCmd where Wild presented for discussion a specification requested by MatCmd covering the XO-1430 with a right-angle gearbox. Tilley received additional items the specification should have listed, including predicted altitude power curves; the specification returned to Detroit for modification prior to formal submission to MatCmd.

Right-angle gear drive construction details were discussed. Douglas Aircraft had sent a telegram stating the reduction gear ratio should be 3.0:1 instead of 3.5:1. In this connection a discussion arose about making the gearbox with a straight 2.0:1 planetary bevel with a 1.5:1 right-angle drive. However, MatCmd thought it extremely desirable if the two optional ratios of approximately 3.0:1 and 2.5:1 were retained, and depended on which way the epicyclic gear was assembled into the housing. The drawings included a thrust bearing resolving propeller shaft radial bending loads directly to the wing extension that formed the drive housing. It was stated that a single bearing at the propeller cone mounted in a way that carried propeller shaft bending loads back to the gearbox driven shaft thus would require only one universal gear type at the propeller shaft gearbox end. MatCmd recommended mounting this outboard bearing in rubber so that bending stresses would be relieved by rubber deflection and thrust loads were taken directly through the rubber in compression.

Continental inquired as to whether an SAE No. 50 or SAE No. 60 propeller shaft should be used. Lt McCoy of the Aircraft Branch stated that while a No. 60 propeller shaft would increase the engine weight only slightly, the installation weight would be considerably increased because of the heavier No. 60 propeller hub and larger blade shanks; the No. 50 shaft would be ample for the proposed 3.0:1 reduction ratio.

The hydraulic propeller control was discussed and it was decided that due to the dual use of the epicyclic gear as a XO-1430-1 reduction gear as well as the Douglas Aircraft installation, provision should be made in the reduction gear housing to incorporate an optional propeller governor and that the thrust bearing housing should also include an independent optional governor oil supply to provide for the use of low-viscosity propeller actuation oil. MatCmd also wanted the design to be coordinated with the Curtiss-Wright Corporation Propeller Division and the Hamilton Standard Propeller Division.

Continental proposed mounting the epicyclic stationary gear by means of two torque arms extending horizontally in either direction to oil control pistons, which would serve as a torquemeter. Continental thought this would add very little weight and would enable torque to be monitored regardless of how power was absorbed and regardless of whether the engine was on the test stand or in an airplane. MatCmd proposed a slight modification to accommodate propeller rotation in either direction, along with the recording of torque delivered by the propeller back-driving the engine. MatCmd suggested that machined mounting pads be supplied at three points on the gear box to permit attachment of any type of mounting bracket incorporating rubber mounts. Details of epicyclic gear train mounting were discussed and all concluded that a very simple and satisfactory reduction gear would result from the design. Continental promised to work on this project as rapidly as possible and deliver reduction gear drawings at the earliest possible date so that XO-1430-1 development testing could proceed. [USNARA RG342 P031074. 22 Oct 1938 Memorandum Report E-57-285-90, Conference with Representatives of Continental Motors Corporation on the O-1430 Type Engine.]

28 Oct 1938. Prescott visited Continental and reviewed the new XO-1430-1 bevel reduction gear in final preparation for fabrication before development test continuation under Contract W-535-AC-8131. Mr. Lamb of the Gleason Gear Works was present and stated that the design's gear mounting and stresses were satisfactory. He said that work was already under way to make up a test gearset in order to determine the best tooth design in preparation for manufacturing the gears. Forging blanks had been ordered from which these gears were to be made, and detail drawings were being prepared. It was expected that the new reduction gear would be available in about four weeks.

Minor details for the Douglas Aircraft installation were also discussed. The Douglas Aircraft installation would have reduction ratio options of approximately 3.0:1 and 2.5:1, while the XO-1430-1 options would be 2.2:1 and 1.8:1, with the ratio selection would be made by simply turning the epicyclic gear unit end-for-end. This gear design was based on very moderate tooth loading and the design ensured that all teeth contacts carried their proper load proportion. [USNARA RG342 P031072. 31 Oct 1938 Memorandum Report E-57-285-91, Visit of F. L. Prescott to Continental Motors Corporation on October 28, 1938, Regarding XO-1430-1 Reduction Gears.]

14 Nov 1938. Kinnucan visited Prescott at MatCmd and reported difficulty with the present XO-1430-1 lubrication system that fed engine oil to the main bearings and reduction gears through the high-speed accessory drive shaft running the engine's length. The shaft served as an oil cleaner but the dirt removed traveled forward and was discharged into the reduction gear and accessory drive gear bearings. He proposed that the next crankcase be fabricated with an oil gallery from which all engine bearings would be lubricated except the accessory drive shaft bearings in the intermediate diaphragms; these would be lubricated with oil at reduced pressure in separate lubrication system branch common with the rear accessory bearings. During past testing it had been necessary to use external oil plumbing to carry oil to the reduction gear section and this external oil line would be eliminated by the proposed oil gallery. As an intermediate step, he proposed that the present crankcase be modified by drilling through the diaphragm webs near the accessory drive shaft to provide an internal oil passage that carried oil to the engine front.

Kinnucan proposed that a separate diaphragm be incorporated in the engine between the crankcase front and the accessory drive housing; this diaphragm would be doweled to the crankcase front and retained by countersunk screws so that the accessory drive bearing bores in the crankcase front and the diaphragm would be bored in a single operation, eliminating drive shaft misalignment. This construction would eliminate the most serious objection to the present arrangement in which the diaphragm was part of the reduction gear housing; in the original scheme, when the reduction gear housing was removed the accessory drive shafts were left without support. The proposed construction would provide support for these accessory drive shafts when the reduction gear was removed and would also simplify the engine front assembly. Bearing lubrication in this new diaphragm would be address by direct connection with the proposed oil gallery coming from the engine rear instead of being fed from inside the rotating accessory drive shaft. The present engine could not be modified to supply main bearing oil from this gallery, but future crankcases would be designed to incorporate the internal oil gallery with drilled oil feeds to each main bearing.

Prescott observed that the torquemeters for the Douglas Aircraft installation should be designed to indicate torque regardless of the rotation direction. Double-acting pistons in conjunction with a small automatic valve that would select meter indication from either piston side and serve a single gage that indicated torque in either direction. Kinnucan wondered whether about the damping effects of the proposed torquemeter because of the oil dashpot action. Prescott replied that no such tests had been done, but there was also no reason to think such dampening would not exist.

MatCmd personnel had suggested using a controlled oil bleed from the torquemeter cylinders in order to stabilize torquemeter operation. This could be accomplished either with a drilled oil bleed that could discharger the overflow oil into the reduction gearing or by controlling the clearance between the oil pistons and cylinders and omitting the piston rings; the later method was preferable since it would eliminate the piston ring static friction and increase the torquemeter sensitivity.

Kinnucan suggested a single oil supply and high-pressure pump serving both the propeller governor and torquemeter. The torquemeter and average propeller governor oil consumptions would be minimal, so that one small 4 – 5 gpm pump would be adequate for both. Prescott had no objection to this. However, in the case of remote propeller gear boxes, engine oil supplied through long pipes would become cool and too viscous; a separate oil supply for these remote gearboxes was preferred. Kinnucan planned to study a separate propeller oil supply for the Douglas Aircraft right-angle drive; it appeared that a relatively large oil tank would be required adjacent to the thrust bearing housing in order to completely separate the propeller governor oil system.

Kinnucan wondered if providing governor drives for all reduction gear ratios was advisable if it involved a slight engine nose length increase. Prescott said that MatCmd thought all proposed reduction gear ratios should be accommodated, even if the nose case length increased.

Kinnucan and Prescott brainstormed ways for changing right-angle gearbox rotation. One method was to provide two positions for the large bevel gear so that rotation would be determined when the gearbox was assembled; engine rotation would remain the same in either case. The spiral bevel gears would be designed to oppose the end thrust due to the tooth spiral angle against the thrust due to bevel gearing. In this case a 6-lb weight savings would be made in the thrust bearing used to locate the gears. An alternate proposal would design the gearbox to use Zerol gears, in which case the thrust on the bevel gear bearings would be the same for either rotation direction. However, no machines were available that could cut and grind Zerol gears this large; they were under construction and would be available in the near future. Consequently, the first right-angle drive would incorporate spiral bevel gearing and be designed for only one rotation direction; in the future Zerol gears would be used to provide optional rotation. Kinnucan stated that parts for the new XO-1430-1 reduction gear were being expedited and Continental expected the engine to be ready for operation prior to 15 Dec 1938. [USNARA RG342 P031063. 17 Nov 1938 Memorandum Report E-57-285-92, Conference with Mr. Kinnucan of Continental Motors Corporation on XO-1430-1 Engine.]

10 Dec 1938. Kinnucan met Prescott at Wright Field to continue the XO-1430-1 lubrication discussion they had begun during the 14 Nov 1938 meeting. Instead of the separate oil gallery, it also appeared possible to modify the accessory drive shaft forward end by installing a central tube that formed a step to prevent sludge from being ejected at the extreme forward end. This tube would be installed at the first main bearing diaphragm so as to feed the front accessory bearing with oil picked from the accessory drive shaft center. This scheme would make it unnecessary to install external oil plumbing or an additional oil gallery. The new accessory drive shaft construction also eliminated the former defect of blind-ended holes so that pressure oil could be fed to the bearing center and flow both ways, thus ensuring oil flow over the bearing surface and eliminating the dead regions where there was formerly no oil flow. Kinnucan planned to modify a spare accessory drive shaft and test it at the first opportunity.

XO-1430-3 Specification 1002-C

Reduction gear spider lubrication was reviewed and a satisfactory arrangement was agreed upon whereby the oil passages leading to the spider arms would be of adequate size without reducing the driving spline strength in the spider hub. The retaining nut for the driving member attached to the reduction gear driving shaft employed a stamping with a single lug engaging one spline tooth as a locking method; the single lug was breaking off. Prescott suggested that the lug number on the stamping be made equal to the number of spline teeth and thought this would be adequately strong to lock the nut.

Kinnucan and Prescott reviewed drawings for the XO-1430-1 and XO-1430-3 torquemeter installation. It was proposed to install an emergency stop at one side in order to lock out the torquemeter. However, upon examination it was discovered that this locking method would impose undesirable side loads on the reduction gearing. As a result of the discussion, it was proposed that some form of stop such as a high-rate spring be installed, which would form an emergency stop and provide some resilience in case oil pressure failed. Prescott and Kinnucan discussed methods to make the torquemeter automatic and fully reversible without changing any parts. The proposed metering device had a steel part that metered against cast aluminum and it was thought that inserts should be used in all such locations to eliminate erosion that would certainly occur under the high pressures encountered during torquemeter operation. An alternate form was discussed in which the torquemeter outlet oil escaped from ports in the side of torquemeter cylinder walls. In this case the torquemeter pump would be called upon to supply only the pressure necessary to resist the torque in the meter itself, and a fixed oil quantity would be forced through the torquemeter cylinders. Kinnucan said this operation method would be studied. It was also agreed that torquemeter completion was secondary to the gearbox development and should not delay resuming XO-1430-1 testing. Kinnucan stated that considerable difficulty had been encountered in the reduction gear housing design, and as a result of this conference another housing design change would be incorporated. Therefore, it was impossible to resume XO-1430-1 testing until about 1 Jan 1939. [USNARA RG342 P031065. 13 Dec 1938 Memorandum Report E-57-285-93, Conference with Representatives of Continental Motors Corporation on XO-1430-1 Engine.][USNARA RG18 NM53 E268 B2. Air Corps Specifications - Engines - XO-1430-3.]

Send mail to with questions or comments about this web site.
This website depends on cookies to make it function. If you continue to browse, scroll, click or otherwise interact, you are providing implicit acknowledgement of and agreement to this.
Copyright © 2002-2026 Aircraft Engine Historical Society, Inc.