Bonneville 120R

[mr bidd]

Hello, I am new to this site and the classic bike scene.  I have recently purchased a 120R and I have done quite a bit of work to it like new clutch, new primary chain and adjuster, new brakes, tyres, tappets, full service etc.etc.  Took it for it's first major run and found out that at 4000 revs I was only doing 68 mph.  This seems very low geared to me.  I have checked the sprockets and it is running a 21 tooth front and a 47 tooth back.  I have been told that I can go down on the back one tooth only because of the sprocket fitting around the drum.  Can I gear this bike up any other way?  Can anyone tell me what revs are you doing at 65mph?  Thanks  Chris

[L.A.B.]

Welcome mr bidd

Took it for it's first major run and found out that at 4000 revs I was only doing 68 mph.  This seems very low geared to me.  I have checked the sprockets and it is running a 21 tooth front and a 47 tooth back.  I have been told that I can go down on the back one tooth only because of the sprocket fitting around the drum.

Your T120R already appears to be somewhat overgeared from standard, as the original T120R final drive gearing was 19F/46R, which gave a theoretical road speed of approximately 62 MPH at 4000 RPM.

[mr bidd]

Thanks for that information.  So you are saying a standard 120R will only cruise at 60mph?  I know mine has a bigger front sprocket than standard as it says 19 and 46 in the manual.  So from what you are saying I won't be able to gear it up any more? Thanks  Chris

[L.A.B.]

So from what you are saying I won't be able to gear it up any more?

Unfortunately the main limitation on how big a front sprocket you can fit is governed by the size of the access hole behind the clutch.
However it is possible to cut notches around the edge of the access hole to clear the teeth of an oversize sprocket.
I also believe it is possible to fit an oversize sprocket if the gearbox mainshaft is removed first, so that the sprocket can be slid into position from the back of the primary case?

Another possible option would be to see if you can find a parts supplier that has a larger engine (crankshaft) sprocket, if such a thing exists these days?

Some belt drive conversion kits are also slightly higher ratio than the standard primary ratio, I believe?