BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: maddogmargetts@yahoo.co.u on 17 October, 2021, 09:48:42

Title: M21 rear sprocket change
Post by: maddogmargetts@yahoo.co.u on 17 October, 2021, 09:48:42
I wonder if it is possible to alter the gearing slightly on my M21 by put a sprocket on the back wheel with less teeth. Would this give the bike a bit more speed? I don't want much more it's just that I can move off happily in 2 nd gear as 1st is for climbing trees and as the bike is solo is more or less redundant.
Title: Re: M21 rear sprocket change
Post by: idie on 17 October, 2021, 10:01:39
Will be hard to get a smaller rear sprocket because it is part of the drum. Much easier to change the engine sprocket, go up on one or two teeth.
Title: Re: M21 rear sprocket change
Post by: Jim S on 17 October, 2021, 14:15:32
I went from 46 to 42 teeth on the rear sprocket of my B33. 42 teeth is the standard configuration. It made a huge difference. Engine revs are 10% lower. I can easily ride at 50 mph without worry about high engine revs.
On my bike, the sprocket and drum are also integral but it my be harder to find an M21 part. I bought mine from British Cycle Supply in Nova Scotia.
Title: Re: M21 rear sprocket change
Post by: maddogmargetts@yahoo.co.u on 22 October, 2021, 17:19:57
thanks for the help I shall probably take the easier option and change the front sprocket.
Title: Re: M21 rear sprocket change
Post by: maddogmargetts@yahoo.co.u on 22 October, 2021, 17:30:20
while changing the sprocket is it possible to renew the oil seal at the end of the shaft as well or will i have to take the gearbox apart to do this?
Title: Re: M21 rear sprocket change
Post by: idie on 22 October, 2021, 20:05:05
Very confusing, by front sprocket do you mean engine sprocket or gear box. Gearbox sprockets you can only get an 18t and that's from India. I would go for the engine sprocket and a bigger choice. No seal on engine. Gearbox one can be changed.
Title: Re: M21 rear sprocket change
Post by: Rog1 on 22 October, 2021, 20:19:34
Do you mean (looking at your first post) your M21 is on sidecar gearing?

It's better to change the engine sprocket because if you put a different size sprocket on the rear chain, it will make the speedo less accurate. Changing the engine sprocket might mean you need a longer primary chain, though.