B40 gearbox sprocket

[Greenfield]

Has anyone come across a 20T gearbox sprocket for a B40 (1962)? The standard one is 19T pt no 40-3123 but BSAUNITSINGLES shows a 20T one as being 'British made'. I'm wondering whether anyone has experience of fitting one of these and a possible supplier.

[Spaceman]

The 20T sprocket is 40-3124 but I can't find anyone who still does it. Out of interest, why do you need the bigger sprocket - it's not as though the B40 generates that much torque to start with?

[Greenfield]

Thanks Spaceman. A wet and thoroughly miserable day has confined me indoors and whilst reading about a range of BSA issues, it got me thinking about gear ratios and sprocket availability. There is a whole range of sprocket combinations for A50/A65s and several gear sprockets smaller than the standard 19T in the B40, so it got me wondering why there wasn't a bigger sprocket made.
Question answered. So, there was a 20T produced which must be fitted to some bikes out there. Somebody must have thought it a worthwhile modification for it to go into production.
Cheers

[Spaceman]

The 20T sprocket was fitted to the B40 Enduro Star that was a trials version of the B40 which also used a 52T rear sprocket rather than the B40's standard 46T sprocket - this had the effect of lowering the Enduro Star's overall gear ratio. I'm not sure that a standard B40 fitted with a 20T gearbox sprocket would provide any real advantage - it would just reduce the amount of acceleration available and might even reduce its top speed!

[Greenfield]

The enduro star is not a model I know much about, and after rooting around in the members area I'm not much wiser. The parts book gives the same engine and clutch sprockets as the standard B40 but surprisingly the enduro has a different part number for the locking washer for the gearbox sprocket.
Using Spaceman's gearbox and rear sprocket sizes, it would give the enduro an overall gear ratio of 5.878 compared with 5.778 for the SS90 and 5.474 for the standard B40. Ok, perhaps a bit nerdy, but someone might be interested.

[Spaceman]

The Enduro Star also used the same top gear pinions as a standard B40 and it was only the gearbox and rear sprockets that gave it a lower 4th gear ratio compared with a standard B40. Unlike the weak swan neck frame used by the B40, the Enduro Star used a welded duplex frame similar to that used on the B40WD and the later B44. I suspect that the larger rear sprocket (52T) used on all these welded frame bikes may have had something to do with providing enough clearance between the swinging arm pivot and the rear chain. Once the rear sprocket size had been determined, BSA had no choice but to use a 20T gearbox sprocket to give the Enduro Star a slightly lower overall gear ratio for trials purposes compared with the standard B40.