Hi,I'm having trouble in obtaining a 19t gearbox sprocket for my 1955 s/a b31, there are sprockets from places such as India but mostly 18t, could anyone advise on other sprockets may fit, my sprocket size should be 5/8×1/4 as rear chain,I have other sprockets which I think for A10/A7 but these are to thick, any advise would be appreciated, thanks in advance.
Hi,
Try C&D Autos. They do not have a website but advertise in The Star. Their number is 01564 795000.
Dave.
Is that in the UK
Hi,
Yes. They will post to anywhere.
The B25 uses a 1/4" chain but I do not know about the spline for the shaft.
Dave.
Thanks Dave I'll try ringing tomorrow.
I did a search on the BSA part number of 42-3016 and found the below which I hope helps.
https://www.britishbikebits.com/19t-gearbox-sprocket-bsa-b31-b32-b33-b34
What is your rear sprocket like? The 1/4 chain and sprockets wear fast and if both sprockets are on the way out it may be easier to go up to the later chain.
Quote from: limeyrob on 21 Mar, 2026, 10:50 What is your rear sprocket like? The 1/4 chain and sprockets wear fast and if both sprockets are on the way out it may be easier to go up to the later chain.
I use the 520 chain on my Gold Stars (DID x-ring chain) with 1/4" sprockets and no issues with excessive wear (yet!)
My Ducati Supersport (117bhp) and my Ducati Scrambler (74bhp) both use 520 chains, they have o-rings though. Chains and sprockets last around 17,000 miles.
Its years ago but I did many miles on a 55 B31 with the 1/4 chain. I did find it needed more regular adjustment than the larger chain on the A10 and suspect that's why BSA changed. My sprockets were wearing and had I not sold the bike I would have gone for the A10 set as it seemed to be easier to get the rear drum/sprocket.
Upgrading to A10 sprockets and chain crossed my mind but already having purchased 1/4 chain I would rather stick to original at the moment,rear sprocket is good,
Having mentioned earlier I had an A10? 19T gearbox sprocket I decided to make that fit my 1/4 rear chain so on the lathe it went I took off just under 1/8" and after dressing teeth it fitted perfect so now just have to fit primary chain and rear chain.
In my experience gearbox sprockets tend to wear out twice as fast a rear sprockets. Not surprising when you consider that it is turning about twice as fast so each tooth engages with the chain twice as often.
I always thought BSA had made a mistake using the smaller chain for these years. The rear chain runs about twice the tension in top (and a lot more in lower gears) than than the primary so it always made a lot of sense running a larger drive chain than primary on the basis of chain stretch and sprocket wear, and by 58 BSA came to the same conclusion.