In the box of bits for my '58 M21 project I have 3 X 2 lobe engine sprockets - all of which look to be in good order. One has 16 teeth, another has 18 teeth, and the last one has 19 teeth.
According to my parts book, the 16T is for M20/21 sidecar, the 18T is for M20 solo, but it lists a 20T for M21 solo (the only one I don't have) oddly enough the 19T is unlisted!
I'm minded to use the 19T one anyway, does anyone have any experience of this or want to do an exchange?
Gary 
email (option): gj.owen@hotmail.co.uk
BSA made engine sprockets from 16 to 24 teeth...
You should also have a 19T gearbox sprocket fitted to your model...
Altering the engine sprocket size is the way BSA raised or lowered the gearing for different uses.
Very low gearing (16T) would be used for sidecar work to help the bike pull away and climb hills whilst pulling the additional weight of a sidecar etc.
The price for using such low gearing is that top speed is restricted and engine revs are higher for a given speed...Similarly low gearing would be used for slow 'off road' work with a trials machine...
The gearing selected originally is a compromise chosen to give reasonable all round performance whether solo, with a sidecar etc...
My preference is for the characteristics of a bike with higher gearing which maintains lower revs at cruising speeds.
A change to one tooth less than standard will lower the gearing but the bike will still run fine like that..Ultimately it's more a question of how you like it when you eventually ride it and the type of roads you will typically use it on.....
For now go with the nearest sprocket you have to standard, either up or down in size and you can assess it once you are riding it...
Alternatively, the full range of sprockets are available new if you can't get a swap....Ian
email (option): ian@wright52.plus.com