General => Douglas Motorcycles - General Discussion => Topic started by: RobManley on 01 Jun 2016 at 09:29

Title: Aero gearbox woes
Post by: RobManley on 01 Jun 2016 at 09:29
My gearbox was noisy is anything but 4th gear, and then in an unrelated event, on the return from a trip it would lock in first gear. 
So, the bike came apart and and hasn't been looked at since.  Until now, 10 years later.

Looking at the gears on the main and lay shafts, the fit is poor, there is major play and wobble of the gears on the shaft and looking at the gear teeth themselves, some are very worn with poor surface finish.
Being a grease filled gearbox I would expect a little play between the gears and the shaft but not as much as I have and I cant really do anything about the worn teeth. 

So, really all I can do is put it back together and live with it.  But - I am still curious to know how much 'wobble' there should be, if any.


Link to video of play: (
Title: Re: Aero gearbox woes
Post by: Doug on 01 Jun 2016 at 17:09
The gears are a little on the loose side, there should be barely perceptible rocking.

The gearbox should have a supply of self-leveling grease, that should not require clearances much more than heavy oil. Often owners substitute regular grease to try to prevent it from leaking out of the gear case. That does not get into all the journals until they are good and hot and it liquefies the surrounding grease, by which time some more accelerated wear has occurred.

Perhaps you can sleeve the gears, but more than likely a combination of wear has occurred on both the shaft and the gears.

Title: Re: Aero gearbox woes
Post by: RobManley on 02 Jun 2016 at 08:10
Do you know if any of the original drawings have survived?  It is fairly easy to determine the original size of the parts but knowing the tolerances they were machined to would be very interesting...

Im not sure how feasible it would be to sleeve the gears as they are pretty hard and it would mean grinding down the shaft or the bore of the gears to get any sleeve in there.  Then there's the fit of the splined gears which is just as bad. 

Did Douglas make the gearbox themselves on this model?

Title: Re: Aero gearbox woes
Post by: Doug on 02 Jun 2016 at 19:02
No gear or shaft drawings survived for the Aero models that I am aware of.

The gearbox is of Douglas' own design and manufacture. The 4-speed box was introduced c1932 as an option, and there are three similar variants that you need to be aware of when mixing parts. One included a change to the tooth form, so you cannot swap individual gears.