A, B, M Gearbox main shaft clutch sleeve taper differences

[watts]

I am trying to understand the changes and/or evolution of this.

My interest started when I tried to find a 6 spring clutch sleeve.  I have several now with at least 3 types.

Is it right that all post war rigid plunger frames have same gearbox and mainsheet clutch taper?

Reading around I have seen mention of thick and thin ones?

From late 1940s to late 1950s there seem to have been 3 types of pre unit 4 speed box.

I have been reading what I can but not entirely sure I understand or just missed something.

3 types of gearbox

To 1948 ish
then 1948 to S/Arm
S/Arm on.

Does each of these have the same main shaft and taper or is variation in each gearbox type?

It is a clumsy question which illuminates my ignorance.

I also know BSA were notorious for using up bits so there maybe no definitive answer.

Thanks

Matthew

[Derek996]

My bike is a 1948 ZB32 and has the 1946-48 lightweight gearbox. I don't know what the taper is but I do know that none of the later clutch sleeves will fit. The sleeve has an internal taper for a puller unlike the elater ones which have an external taper.

[JulianS]

The B and M series plunger and rigid used gearboxes with the same mainshaft (15 4260) from the 1949 season on and it was used right until the end of M21 production. That mainshaft was used in various boxes from 1936.

The difference in the clutch sleeves may be accounted for by clutches in different models having a  different number of plates or sealing arrangements in the chaincase or different chain lines.

The clutch sleeve 65 3873, shown in the 1949-53 M parts book, was also used on the swinging arm B31/33 Gold Star and twins. However a different sleeve (42 3107) was introduced in 1956 on the B31/33 and A7/10. This, together with a spacer between the crankcase and front of primary chaincase and a change to the cush drive moved the clutch and chain line outwards by about 1/8 inch. This was to give clearance between gearbox casting and the tin rear chaincase introduced in 1956.

This new sleeve was not fitted to the Gold Star or the M series which keep their original parts.

If the 4 spring clutch is fitted in place of 6 spring originally fitted with the sleeve 65 3873 the clutch and engine sprocket may be out of alignment. Some dealers supply a version of the 4 spring sleeve with a larger taper which rectify this problem.