Author Topic: The Heavyweight Aero models, 1936-38  (Read 1252 times)

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Offline Doug

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The Heavyweight Aero models, 1936-38
« on: 07 Jun 2021 at 05:31 »
Aero Heavyweights 1936-38

When Aero Engines Ltd took over from Douglas mid-1935 they inherited ten models ranging from 150 to 750cc in side and ohv configuration, one even being a 2-stroke single. This was quickly whittled down to four side-valve models; a 250/350cc and a 500/600cc. Given that most of the machine tools had been sold off, they likely had little choice. While the lightweight twins were a continuation of the previous year’s model – indeed a gradual and unbroken evolution since the EW – the 500/600cc was closely based on the Blue Chief that had been newly introduced in 1935.

The 1935 Blue Chief catalog image

To a certain extent the Blue Chief had itself harked back to earlier times, and broke away from the S6 and its derivatives that had defined the heavyweight side valve models since late 1929. Particularly in regard to the cam and tappet layout, replacing Freddie Dixon’s roller cam followers of the S6 with the preceding and much simpler direct tappet arrangement used on the 350/600cc EW. (The lightweight twins had never abandoned the EW cam layout.) The distinctive large, finned oil sump had already retired with 1933, and like the other 1934-35 big twins, the Blue Chief used a dry sump system with duplex gear oil pump to move the oil around and back to a compartment in the petrol tank. Yet further cost cutting was needed for 1936.

The 1936 Aero 500/600cc catalog image and specifications

Gone was the 1935 option of a quickly detachable rear hub or optional foot shift gearbox. An opportunity to simplify the frame was taken, and gas welding replaced several lugs; the most apparent on the front down tube at the tank rail and at the bottom where it joined the engine bearers. However it was in the engine the greatest number of changes occurred. The geared oil pump that fed a drilled crankshaft was replaced by a simpler reciprocating pump. Metering the oil via an oil sight glass was an anachronism, but the oil did not complain and continued onward to a spray bar that dripped oil in the path of the crankshaft and conrods. This system had been in use on the 250/350cc since the 1931. Drillings allowed oil vapor into the tappet chests and pooled oil back to the timing chest (the tappet chests were slightly more oil tight than the prior lightweights enjoyed.) A large vent tube from the front tappet chest carried any oil still animated across the top of the engine to the front of the primary chain case.

The early 1936 Aero timing chest and oil pump

Other cost cutting measures were the abandonment of the Lucas magdyno used for 1934-35 and return to the slightly less expensive BTH M2 magneto and feeble BTH pancake dyno (Series PA, to give them their correct title). They moved the dynamo to a lower position on the timing cover than used on the 1930-33 models. Just outboard of the timing pinion was a second pinion that now engaged the dyno gear. Lighting switched from Lucas to Miller; a 6-1/2 inch No.74ES head lamp and a 35E tail lamp.  Perhaps the most dramatic reversal was the abandonment of aluminium cylinders and cylinder heads (debut on the Blue Chief and the similar engine of the Endeavour) for more conventional cast iron.

The petrol tank scheme was the same as used in previous years, a dignified black with side and top panels of Douglas blue surrounded by a gold pinstripe. The option of chrome plating instead of black was not offered. Though the tank retained the same shape, it was without the expensive instrument panel containing Lucas ammeter, lighting switch, panel/inspection lamp, and Eureka oil pressure gauge of the 1934-35 big twins. A colorful ‘winged’ transfer with “Aero” and the engine capacity in a center shield added a bit of decoration to the top panel. Pride & Clarke, who were the sale agents for Aero Engines (Douglas), had a nearly identical transfer with their name in the shield that they presumably applied to other marques they were selling. The exact placement is unknown as period photos of the tank top have yet to come to light as have any survivors with traces of the transfer. Drawings for the transfers survive as well as its listing in the petrol tank group of the 1936-37 spares list (i.e., it was not intended as a rear mudguard transfer). It is possible the use of the transfer was short lived.  One filler cap was engraved “OIL”, with recommended brands in a tri-lobal arrangement (carried forward?) All in all this made for a very conservative machine, sold through the sales agency of Pride & Clark. Left over Endeavours (and probably any other 1935 machines that could be assembled from inventory) were also sold at reduced prices. While not particularly fast, the 1936 500/600cc Aeros and the even less powerful 250/350cc models represented good value and were probably about as simple and reliable a machine as Douglas had made since the EW.


The first change occurred before 1936 was out. While I use mid-year to describe this, it is not really known just when during the year the change occurred and indeed the various changes mentioned below likely occurred at different times.

When first introduced, the crankshaft nut incorporated a face dog that drove an oil pump worm; the worm free to spin on a coaxial post pressed into the timing cover. The oil pump was also mounted on the inside face of the timing cover. (See image previous images.) From this scheme, the timing side shaft of the crank was lengthened to reach all the way to the timing cover where it was supported in a bronze bush. A gutter was added to collect oil running down the inside face of the cover, whence a hole lead it to the bush. The oil pump worm now resided on this elongated timing pinion shaft and was keyed directly to it. Presumably this was done to provide more support and eliminate the extra manufacturing steps of the drive dog.

The mid- to late- 1936 crankshaft end and inside face of timing chest cover.jpg

At the same time, a slot that had allowed oil mist from the crankcase to pass through the timing side ball bearing into the timing chest was eliminated. Two 5/16 inch holes remained as before through the crankcase wall connecting the crank chamber to the timing chest. One to allow the oil pump spray bar to project into the crankcase chamber, the other above the oil weir level to allow oil mist from the crankcase into the timing chest. The elimination of the slot might have been to increase the pressure differential, and so encourage more oil to escape through the hollow cam spindles and lubricate the cam bushings. Or maybe it was vestigial feature for a timed breather like used on the lightweights. (I do not know if the Blue Chief may have had such a breather.)  The first ’36 engines had a crankcase breather that vented via a one way disk valve (in the vent pipe fitting?). With the ‘long-crank’ ’36 engine the one-way valve was eliminated. The breather pipe changed also, some factory drawings refer engines as “Series 3”.

In the gearbox there was a change to the lay shaft and a ball race, and elimination of a floating bush, bearing bush and washer. The opportunity to disassemble, compare, and photograph the two types has not arisen.

Before the year was out, the tubular cast aluminum silencers introduced in 1935 were replaced by a chromium steel silencer of similar cylindrical shape, but the ends caps were conical rather than spherical. Internally a spiral baffle was used, whereas the ’36 models had the extension of the exhaust pipe capped and multiple slots cut into the side of the pipe to disperse the gas into the silencer chamber. This had been a traditional Douglas design since the late twenties. Some of the last of the long exhaust pipes may have had simpler to manufacture radial holes rather than slots, before changing to the baffles incorporated into the silencer body proper. The clip on, decorative fish tail was also eliminated.

There is an oddity in the 1936-37 spare list regarding petrol tanks. It should be first explained that the list covers several models from 250 to 600cc and two model years. To save space, each model is identified by a letter – A, B, C, D, etc. – and the group of letters after each part shows which model(s) that part is used on. This practice of using code letters in their spares lists combining several models dates back to 1930. At the front of the list would be a key stating the model(s) associated with each letter. Except the 1936-37 spare list does not have a key! Instead there is a space where it seems the owner’s particular model would be inked in. There are six models, represented by letters A through F. By easy deduction it can be shown that A, B, C, & D are the 250, 350, 500, & 600cc respectively of 1936. F is code letter for the 600cc of 1937. This leaves E which based on crankshaft and oil pump components must represent the mid-1936 ‘long crankshaft’ 600cc. Now with that out of the way… Initially the same tank (part number C/17949) served the 500 & 600cc models; that is, followed by code letters C&D. Then a later petrol tank (C/18301), with a letter code E. [We know it is later because from c1924 onward Douglas assigned part numbers in chrono-numeric order. The lower the number the older it is; and higher the number the more recently it had been issued. By the time of the 600EW they were assigning part numbers in the nine thousands, the S6 the eleven-thousands, and the Aero the sixteen-thousands. Assemblies, including permanent assemblies, were prefixed C/ or CA/ and revisions to a part number by a suffix -1, -2, -3, etc.] It seems improbable the changes internal to the engine caused a change to the petrol tank and it is presumed the change was unrelated other that occurring about the same time; i.e.: mid-year. What did change mid-year to warrant a whole new part number (rather than a revision) is presently a mystery. Nor does this represent the change to the 1937 tank, as that has its own part number (C/18529), only used on the 1937 600cc (code letter F), and has the helpful description “chrome black pan[els]”.


1937 Catalog image

The 250/350cc models might have been a little too anemic or sold poorly, as they disappeared from the 1937 lineup. As did the 500cc; the 600 was the sole remaining model. Cost cutting measures were evident on the 600 model. Tubular headlamp stays were replaced by 3/4 x 1/8 inch flat bar. The subtly shaped cast aluminium covers for the tappet chest were replaced by flat rectangular plates of 3/16 inch thick steel. These were still painted black, at least in catalog images, so the material change does not seem to have been to allow for dressing up with chromium plating. The covers were now retained by a stud and nut, rather than by a bolt. The cylinders and heads were painted silver; perhaps to make them look like aluminium, but they were still cast iron! The ’36 magneto sat on an aluminium spacer, possibly as an expedient solution to the difference in spindle height between the Lucas magdyno and BTH M2, but also it was relived to provide a bit of an air gap between the crankcase and magneto. For 1937 the magneto sat on two curbs cast integral with the top of the crankcase. The space between the curbs providing the air gap.

The crankpin race changed significantly enough to warrant a part number change (rather than a revision), but while details of the later race (#18471) are known, it has not been possible to compare one to the earlier race (#16829) to see what the difference was. Likewise the crankpin bolt (called “set screw securing webs” in the spares lists) changed from #16831 to #18523. Yet they were able to continue using the same old locking tab washer. The 1936 pistons had one compression, one scraper, and one slotted ring. The gudgeon pins had soft end pads. For 1937 this was changed to two pressure rings and a slotted ring, The gudgeon pin was now retained by circlips. This – of course – required a change in the piston part number. The head gasket came in for a revision. The clearance diameter for bore decreased from 3-1/16 to 2-15/16 inch. This made it a closer fit to the bore diameter of 74mm. At the same time the ‘throat’ between the bore and the valve head pocket increased substantially from 1-5/8 to 2-1/32 inch. Yet there was no known change to the dimensions of the cylinder or head castings.

The primary chain case got a major alteration. This had been an aluminium casting that enclosed the rear wall and sides, with a cast aluminium lid to enclose the face, similar to what the Blue Chief had. The inner portion was trapped in place unless one removed the flywheel and transmission input sprocket. While this could not have been required often, they changed it so the case was made of a fore and aft potion, bolted together at a vertical joint. The fore portion surrounded the engine sprocket and could remain more or less permanently behind the flywheel while the aft portion could be easily removed for maintenance. But when the face and sides of the aft portion became one, it lost its rear wall.

1936 chain case

1937 chain case

1937 chain case, rear side

In that regard it was a retrograde to the earlier sheet metal primary covers (like the EW) that were also open to the rear (towards the engine/transmission). There is indication a decorative trim cap was intended to cover the clutch adjusting nut, but if so it did not make it into the parts lists. The rear chain guard also was changed for no apparent reason. The crown radius decreased from 25/32 to 5/8 inch, with some minor changes to the mounting. Perhaps reflecting a change in supplier? Also there were some changes to the front and rear mudguards, stays, and some of the mounting details. The spares list indicate such, but a detailed study of surviving machines has yet to be done to figure out exactly what the changes were. One such change that is described in the spares lists is the elimination of a “v-washer” for mounting the front number plate. This would suggest a ribbed mudguard was employed, yet I am pretty sure the 1936 models continued with the same non-ribbed profile that had been used on OB, OC, 600EW, and S6. Some of the 1934-35 heavyweight models used a ribbed mudguard but so far no evidence has come to light that they used these in 1936. For the same reason (whatever it was), the rear number plate changed as well.

Primary and secondary chains acquired new part numbers, though the reason for is unknown. Unlike older spares list, they do not mention the size or number of links, but given there was no change to the sprockets this would not have been the reason. A 17 tooth gearbox sprocket for the 500cc disappeared (with the 500 model) from the spares list, leaving just the 19 tooth sprocket used by the 600cc. Though presumably one could still get a 17T, if say a sidecar were fitted.

The gearbox for ’37 also acquired a new prefix code. This was more than just incrementing the code for the model year. Up to this time a thixotropic or self-leveling grease had been employed, injected via a grease fitting near the top of the box. The 1938 edition of the Pittman Handbook of the Douglas advises owners of the 1937-38 models use one of several heavy gear oils. The grease fitting is history, replaced by a small filler cap stamped “OIL”. The hole for which changed from 1/2-20 to a 3/8 BSP thread. The sealing arrangement does not appear to have been changed to cope with the lower viscosity and if any clearances and tolerances were changed internally they are not reflected in the part number/revisions. The gear case cover changed part numbers also, though what the difference was has not been determined.   

The frame acquired some subtle changes. At some point in the year a simple cross tube with the ends pinched flat became the frame support for the saddle springs. This likely occurred when the supply of the previous castings ran out.

1936 saddle lug

Mid- to late- 1937 onward saddle lug

A rectangular protrusion was added to the base of the headstock casting to serve as the fork stops, just above where the steering damper plate would anchor. Traditionally this was accomplished by small posts screwed into the cross tree of the lower head stem. Consequently the ’37 head stem is not drilled & tapped for these posts.

1936 head stem stop, Stops new and as yet un-painted!

1937 onward head stem stop

Old fashion inverted handlebar levers pivoting at the outboard end of the handlebars gave way to what we now consider a ‘normal’ style lever pivoting at the inboard end. Consequently clutch and front brake cables no longer were internally routed through the handlebars and the ‘bars did not need to be drilled for the cables to exit. The adjustable handlebar clamps still used the u-bolt arrangement introduced in 1934, but the alloy spacer shed a few ounces by being pierced in the middle. This provided a window though which to route the new control cable arrangement. Other changes were more subtle. The rear brake rod was shortened from 15-5/8 to 15-1/4 inches. The clevis end changed too, though not sure of the details. The thread on the brake rod only lengthened a 1/8 of an inch; not enough to warrant a change in the clevis. It still used the anachronistic 5/16-25 thread so it was not part of the switch to more common at the time Whitworth threads that Douglas were starting to phase in.

While the BTH cutout mounting plate attached to the same position on the frame (distal end of the hand change lever post), the plate changed subtly to move the cutout up and closer to the bottom side of the petrol tank; perhaps to get it further out of sight. Both the BTH magneto and the AMAL carburetor were assigned new part numbers for 1937. Exactly why is not know at this time, though it is thought this was more due to a technical change in what was being supplied by the respective manufacturers than a change initiated by Douglas.

The rubber foot rest pads, which had been the same since the S6 models, changed so that the wording “Douglas” was on both the top and bottom. This allowed the same pad to be used on the right or left side, whereas previously they had been handed. It took them six years to notice that economy of inventory! Ironically, the reproduction foot rests available for the thirties Douglases are technically only correct for the 1937-38 models. An archaic, rectangular, leather faced toolbox replaced the cast aluminium style that Douglas had been using for several years that had fit neatly into the triangle of the rear chain stay.

To dress things up, there was a new petrol tank scheme of chromium plate and black side and top panels with a winged “Aero Douglas” transfers in white. The oil cap now had the lube recommendations in a circular pattern around the edge, and probably represented simply what Enots was supplying that year. Or did it? According to the spares lists the same cap was used for the petrol and oil openings from 1936-38, which there was no cap specifically engraved for oil. In lieu, the spares lists specifies rectangular transfers denoting “oil” and “petrol” that presumably were placed adjacent to the caps. The part numbers for these transfers date all the way back to the venerable flat tank 2-3/4hp! Like the winged transfer, No picture or surviving example has yet been seen to verify they actually did this; and if they did it may have been only for a brief period. Conversely there are many surviving examples with the engraved oil caps to indicate these were fitted.

1936 oil cap

1937 oil cap

Caps occasionally seen fitted on restored examples with a raised, embossed word “OIL” are felt to be from an automobiles. An extra threaded fitting at the front, left corner of the petrol tank was eliminated. This may have happened before 1937, but it seems too minor a change to represent the new part number assigned to the mid-year 1936 petrol tank mentioned previously. Access to it as a drain plug was blocked anyway by the location of the accumulator.

1936 petrol tank bottom. Note extra petrol fitting or drain in lower right corner.

Compared to the 1937-38 petrol tank bottom. No fitting at the front corner of the petrol tank.

Whether the 1937 models had a steering damper is open for debate. While it is mentioned in the catalog specs, in some adverts the steering damper knob and the anchor plate are absent. It is reported some frames are not even tapped for the anchor plate bolt so Kingswood may have been trying to do without. Yet other 1937 adverts, such as for the sidecar outfit, do show the damper knob present.

1937 advert

This is a good point to bring up the subject of catalog specs. Changes mentioned above, be they from catalog spec or surviving example of known pedigree, are presented as ‘1937 features’. But in reality some of these were rolling changes. The change in saddle lug, tool box, and silencer likely occurring as existing inventory ran out. This can even be seen in the period advertising. The earliest Pride & Clarke adverts for 1937 blatantly show 1936 machines. By July they were still showing ’36 petrol tanks, toolbox, and silencer, though the cylinders were in the ’37 silver paint scheme, inverted levers gone, and a Dunlop saddle (unpaid bills with Terrys had forced Douglas into receivership!) Granted, the photos would have been taken some months prior to the advertisement appearing in publications. Relying solely on catalog images can be misleading as these often utilized older models or pre-production variants for photographing publicity material in advance of the new model season.


1938 Catalog image

“Aero” and the wings disappeared from the petrol tank transfer, with just “Douglas” within an oval border. It was still chromium plated, but the panels reverted to Douglas blue with white pin striping of an entirely new layout. The top panel was abbreviated to just around the filler caps. The construction of the petrol tanks was the same from 1936-38. Indeed they were much the same as the 1934-35 big twin tanks (less the instrument panel), and the addition on reinforcements at the mounting bolts added in 1935. In “The Best Twin” by Jeff Clew, it is stated that when the supply of petrol tanks ran out that Ariel tanks were bought in. However I have not seen any Ariel models of the period that used an oil compartment in the petrol tank. Quite possibly it was that the tanks were made for Douglas by an outside firm, like Ariel.

1938 Petrol tank

The steering damper is back, if it ever left. The cylinders and heads were back to black also. 1938 sees the first mention of a slip clutch into the dyno drive, though it is quite likely it was introduced at some point in the prior year. This was readily accomplished by providing the pinion that drove the dyno with a tapered bore, which rode on a mating bronze hub keyed to the crankshaft. This, and a coil spring to provide some pressure, created the necessary friction to drive the pinion.

1938 Dynamo slip clutch drive

Prior to the S6 models, and indeed continued on the 250/350cc models all along, the pancake dyno had been driven from the magneto gear. As this rotated at half engine speed it needed to be geared-up to spin the dyno at sufficient revolutions. A cone clutch was built into the magneto gear to prevent overloading the gear teeth by allowing a little slippage when accelerating hard; a relative term with the Aeros! Since the big Aero drove the dyno off the crankshaft gear at nearly a 1:1 ratio, they probably felt the slip clutch was not needed. They changed their minds! Otherwise it was believed to be much the same as the previous year mechanically.

1938 was also the first year a stop light switch (and stop lamp) was fitted. A 29E Miller tail lamp was used, which looked like a horizontal cylinder that stretched across the top edge of the number plate. Lugs were welded to the lower chain stays for optional pillion foot pegs (may have actually started in late 1937).

1938 pillion foot rest lugs

A Smiths chronometric speedometer was listed as standard equipment, yet charged at an extra cost of £2-10-0! It did not use the traditional Douglas hub gearbox, rather a right angle gearbox (gearbox and cable likely provided by Smiths) sprouted from the brake backing plate of the front wheel.

For a model (600cc) that superficially just made cosmetic changes to the petrol tank style each year, there was a lot more going on! Anticipated contracts to manufacture the Hispano-Suiza aero engine under license had not materialized, nor was the light aeronautical market ‘taking off’ as it were, so a new 150cc 2-stroke model was added to the 1938 catalog, but that is another story. The eventual outbreak of the war brought down the curtain on Douglas’ last fore and aft twin.

Doug Kephart. Glen Mills, PA, USA
With much assistance from Steve Harle, Norfolk, UK
First published in New ConRod, Jan/Feb 2021 issue.
Revised and expanded 24May21

Offline Doug

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Re: The Heavyweight Aero models, 1936-38
« Reply #1 on: 07 Jun 2021 at 05:39 »
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Offline cardan

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Re: The Heavyweight Aero models, 1936-38
« Reply #2 on: 07 Jun 2021 at 11:17 »
Thanks Doug - very interesting.

You highlight the role of Pride and Clark selling Douglas through 1937. It seems this role was taken over by Commerford's in 1938-1939 - they described themselves sometimes as 'Douglas Distributors'.

There was a Show at the end of 1938, but there was no Douglas presence at all. There was not even an advert by Aero Engines Ltd in the Show edition of the Motor Cycle. Instead, there was a 'positive' announcement in the Commerford's advert in August 1938 to say that 1939 Douglases were to be unchanged from 1938 models, and what a good idea that was too! Consequently through 1939 Commerford's did advertise the '1939 D.C. Douglas', but they also had on offer (confusingly) 1938 models at the same price. I don't know if there was any real production of 1939 models. In late 1939, after the outbreak of war, it was E. Withers, 88, Knights Hill, S.E. 27 who advertised 'THE Original Douglas Factory, trained experts, have the most comprehensive genuine spares stock; free technical advice; trade and retail; post orders executed hourly.'

Pretty grim really - amazing that there was a post-war Douglas motorcycle.


Offline Doug

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Re: The Heavyweight Aero models, 1936-38
« Reply #3 on: 08 Jun 2021 at 05:41 »

Interesting, I assumed Pride & Clarke had the franchise till WW2. The 'no change' to protect the Douglas owner past & future was a certainly tongue-in-cheek. They were hardly in a position to do much more than offer the prior year model! Like you, I have not heard of any 1939 production, and the Comford adverts are the first time I have even seen intention of selling Douglases in 1939. My understanding was industrial trucks (100 per week) and sub-contracting for other aircraft manufactures was the bulk of the work load at Kingswood. A lot of production equipment tooling had been sold off to meet the financial crises of the preceding years and what remained was likely prioritized for the more promising (for future revenue) industrial truck and aircraft component work.

There was drafting work undertaken in late 1937 for the 1939 model year indicating a new 500cc ohv model (pushrod drawing) with shaft drive (output coupling forging drawing) was under consideration. Unfortunately mostly small and insignificant component drawings, though suggesting the overall design must have been well fleshed out by that time to be bothering with nuts, bolts, brackets, and other sundries. Nor is it known if this was even intended for public consumption or was a proposal for the Ministry of Supply (it per-dates the DV-60 prototypes by seven or eight years). That it was Ministry of Supply is suggested by being quite far advanced in preparation, as usually drawings are dated late in the year prior to the new catalog release, not two years in advance. Likely the declining situation through 1938 regarding their motorcycle sales (maybe 350 machines built judging by frame numbers?) scuppered any further work on a new 1939 catalog model and Douglas would not have been able to supply motorcycles in quantity for the military unless the Ministry of Supply gave them a factory full of new machine tools.