Four speed Aero gearbox

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Dear Douglas friends,
I got so much information from this site, that I want to put something by myself into it. I am Gerd from Hamburg and Corona gives me time to work on a Douglas Aero 500 (1936), gearbox 6Q202. Here I want to show the results on an inspection of a four speed gearbox with constant mesh kickstarter. The gearbox worked without mistake. But I found lots of oil inside of the rear mudguard. The outside was clean. I thought, may be the gearbox has lost its oil and became dry now. I checked the oil level by unscrewing the screw to the rear cylinder head (not easy). Lots of oil (grease) was present. Therefore the oil level was ok but the oil showed yellowish flitter in it. I wanted to find the reason for the particels. I had no knowledge on the functions of this four speed gearbox. But I remember my mistake on the bearing bush for the low speed pinion on a three speed gearbox. It gave a full stop during riding (unfortunately to my daughter). This mistake don't want to do again. Removing from the frame and opening was quite straight forward. The picture shows the pinions and the gearbox cover.

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I did not know the action of the different pinions. The picture below shows the low gear position, the moment flow and length of the actuation plunger. The low gear pinion runs on the mainshaft on a bronce bearing, like in the three speed gearbox. In low gear position it becomes coupled to the mainshaft by a dog clutch from the second gear pinion.

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Now second gear: Here the layshaft gives the dog clutch.

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Third gear: The lower pinion pair becomes unlocked and the upper ones receive connection.

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Direct gear is reached by shifting the third gear pinions upwards and removing it from the dog clutch on the layshaft. A dogclutch between third gear pinion and sleeve gear is formed. In my chase the plunger was shifted totally into the gearbox cover, gap zero.

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Inspection is not over now. What I found is, that the low gear pinion had a little bit more play than I would do it. The condition was not serious. The forks showed fresh surfaces especially in the regions, where the teeth could rub. All teeth showed rough surfaces. Sharp edges were present. I looked at the pinions.One had the DK mark on it in combination with D5. The other pinions got stamped with D5 also but not DK visible to me. The astonishing thing is, that bike got low milage after replacing many nuts and bolts. During that restoration the box was opened and sealed afterwars with plastic seal known from other components. If the flash was received over long time, it would have rubbed the forks completely down. I will continue my inspection. Below is the 4 speed spare parts list and I was checking for missing parts.

[Doug]

The gearbox code is confusing. 6/Q... sounds like an engine prefix code, for a 1938 Aero 600cc. Four-speed gearbox codes for the Aero could be M, S, T, or U (note: no preceding numeral.) A Q gearbox code was assigned to the 1935 Powerflow, foot-change.

Prior to 1937 the gearbox used self-leveling grease (grease-oil mixture). Generally the shifting forks do not wear as they are well exposed to lubrication. Unless the grease is too stiff to flow. Then the gears shove the grease aside until everything gets hot and melts some grease. It could be that the gearbox was run for a significant amount of time in the past with grease and only recently changed to an more appropriate grease-oil mixture. In 1937-38 they changed to heavy oil; seemingly with no enhancement to the sealing arrangements.

Usually bronze powder in the lube comes from the bush in the sleeve gear. This is always the most challenging area to get lubrication to. It sees the most wear when in 1st gear and none in top (direct) when the sleeve gear and the mainshaft rotate at the same speed. 

-Doug

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Doug,
the code 6Q/202 is stamped onto the gearbox. It is not corresponding with the engine or frame number.

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The kickstart lever spring unloaded violently during opening. I did not want to get that effect during loading. I removed the bolt, which holds the spring. The shown piece of wood was used as a lever and turned around with pressure to the kickstart quadrant. Reaching the hole in the kickstart cover, the bolt was pressed in the cover. The loaded cover was mounted on the gearbox,  the small tool got screwed to the bolt for reaching the hole and screwing the bolt down. The same procedure can also be used during opening the kickstart cover. One drawback is, I did not set on the small buffer (I feared for my fingers). But that is also possible by using a dummy screw with mounted buffer. The small buffer is less necessary because the lever becomes friction damped during return swing. The large buffer is really necessary. A big dent is present in my cover (no buffer was mounted).
The ratchet in the cover has many actions. The nut on the outside determines the distance between the ratchet rings. Larger gap gives less action angle for the kickstarter. It was screwed fully home, when I dismantled the gearbox.
Don't loose your teeth

Gerd

[Doug]

Gred,

I found a picture of 6/Q146 on a gearbox in a 2011 eBay listing for a 1938 Aero. Engine stamped 6/Q146 (possibly 6/Q140). In 1938 Douglas started the practice of stamping the frame and engine number identical, and it would seem they extended this practice to gearboxes also. Unfortunately all the 1938 Aeros listed in the LDMCC machine registry do not have gearbox codes documented. But it is looking as if the 6/Q code extended to gearboxes also. My guess is your gearbox had been replaced with one from a 1938 model. If so, it was intended to use heavy oil for lubrication.

-Doug

[fix typo. 26Jan21 -Doug]

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Doug,
I have LDMCC machine register, which shows the G/box No (U145) for one machine only.  That code fits into 1936-37 motorcycles system. Did they change anything for the new lubricant?
Gerd

[Doug]

Gerd,

The latest LDMCC Registry I have shows about fifteen U - prefix gearboxes associated with Aeros. So there are a few extent.

No one has made a study of what changes were made to switch from grease-oil to oil. It is possible they did not mechanically change anything; that they wanted a thinner viscosity to avoid wear at the expense of greater leakage. It is clear that they did make a change, as the Pitman series of motorcycle handbooks (the attached image is from the 1942 edition) make specific mention of it. This is further backed up by Douglas eliminating the grease fitting from the filler plug and stamping the head of the plug "OIL".



-Doug

[cardan]

Here in Australia, Penrite have this: https://www.penriteoil.com.au/products/transoil-sae-250

Leon

[Dads bike]

Hi all
Castrol D is an SAE 140 and is still available from Castrol heritage products.

Gerd, looking at your gearbox attached to your machine, difficult to see but the filler plug on the top of the box looks to be the larger hole size associated with the 1937/38 oil filled boxes, is it stamped oil?
Gear tooth form should be involute, to help keep the oil in the box replace the felt seal with a lip seal and finally check the play between the input shaft and out put shaft the bush needs to be phosphor bronze and it should be a running fit ie no play.
Steve.

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Dave,
thank you. It clarifies the changes. I checked the spare parts lists. The 1937 model with the 6P/ engine has the symbol F, as Doug stated. The model F has only a few different parts. If the plug became larger (shown as 18532 Plug, grease gun nipple?) they had to change the gearcase and the washer, as stated. The other change is regarding the layshaft and its ball bearing (H/101).
The 1938 parts list shows the gearbox parts of model F only without changes.
My gearbox has the plug marked by the word oil. Doug thinks, it is a 1938 model due to the gearbox number (6Q/202). Eric, Aero owners know him, has a gearbox marked with 6Q/237. May be ....
What I don't know is, where is the possibility to drain the oil out of the box. It is mentioned in the Pitman book. I have only the inspection plug, which is above the minimum oil level.