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T35 Idler gear bush

Started by douglas1947, 12 Aug 2022 at 16:45

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I have to make new bronce idler gear bushes (camshaft + magneto idler gear).
Does the gear wheel run on the bush or is bush pressed tight in the wheel and it runs wheel spindle?
Or is the bush loose on both, the wheel and the spindle?

A photo with wheel, bush + spindle is added.

I hope someone can help with advise.



There is absolutely no (visible) trace of wear on the pin.
And none on the externals of that bronze bush either !

So why ye say they need to be redone ...
This is from a working engine ?


Hi Michael,

Usually the bush would be pressed into the wheel (light interference fit - say a bit less that 0.001"/0.025mm, or size-for-size + Loctite), after which the inside diameter (hopefully originally about size-for-size with the pin) can be eased out with an adjustable reamer to get a nice running clearance.

That said, some makers did funny things! In a 1910 FN single the big end bush was pushed onto the crank pin, and the bearing surface is between the outside of the bronze and the eye of the con rod. There's also nothing wrong with the floating bush method you mention, except that clearances have to be dealt with very carefully, so it's not manufacturing-friendly.

Don't forget a slot of some kind to allow the oil to get in.



Indeed, on the photo are shown anew bush and a new spindle. But the magneto idler wheel is used and there is a very loose fit to the new bush.
So I think it is important to make a new bush to the idler wheel to get a pressed fit and ream the bush to run on the spindle.
I want to use the old spindle, still in the crankcase.

The same problem I have with the cam idler gear. So 2 new made bushes will do.



Wrong! Think again! The spindle shoud be a light press fit in the crankcase. The bush should be a sliding fit on the spindle. The gear then needs to be a running fit on the bush (dont make it too neat a fit as it is only splash lubricated, so needs enough clearance to allow the oil to get in). The length of the bush needs to be 5 to 8 thou more than the gear. If all the fits are correct, the whole assembly should squeeze up nicely when the nut is tightened, and the gear should spin easiy on the bush. If the bush is left loose on the spindle and the nut not properly tightened, the spindle will work loose and damage the crankcase.

Hope this helps,

  P.S.   The T35 Mk1 handbook has a list of limits and fits for the engine. Whilst it doesn't list the idler bearings, most other steel on bronze plain bearings have a clearance of 2 to 4 thou (on diameter).


Woops, I'm in the wrong era. Sorry!

At least the design is such that everything can be pulled up tight on the bush. I have two engines - the little FN and a bike with an American Spacke De Luxe engine - where a bush is "trapped" in a similar way, but in both cases there are tapers involved, so that a pin has to pull up on a taper and clamp the bush at exactly the same instant. A bit daft...




thanks for your informations.
So I will make the new bushes to fit with the right clearence to the spindles and the bore in the idler wheels.