My guess and it's not a very good guess but here it goes.

I suspect when you fit the clutch nut and tighten it and now nothing will move anymore is due to the gearbox main shaft being pulled over to the clutch side. This will lock up the ball bearing on the other side. It could also be from the clutch sleeve being locked up against the output sleeve gear.

Does the clutch sleeve lock on to the main shaft taper properly or does it run in to the sleeve gear? Try fitting it and the nut without the rest of the clutch and see it it turns.

Are both the taper on the shaft and the taper inside the sleeve gear OK?

Do you have the correct size clutch rollers? If too long it will lock the clutch.

If your Burman expert has never worked on a M series BSA gearbox, I bet it was assembled wrong.


Without seeing the bike, that's all I can think of.

Bruce has mensioned this in one of his anwsers.
Before you blame the gearbox, check that when you tighten the main shaft clutch nut that it is not locking the clutch chain wheel to the centre hub. With the nut tight and no plates fitted you should be able to spin the clutch drum and hold the centre without any drag.