Hello All!
My very good friend and BSA owner has a 1946 B31 that he inherited from his uncle many years ago. We are rebuilding it and has a minor problem finding a clutch for it. Does any of you know were to find the correct clutch for it or if it's possible to use another BSA clutch and how to do this.
It has a very simple basic 6 spring clutch. Used by more models but with more or less plates depending on the engine size. I might have some parts for it, probably not a complete clutch.
The B31 usually has a clutch basket with only three friction plates. There's no reason why the larger five plate one can't be fitted. You can of course fit the Triumph clutch using the right adapter.
I did quite a few miles with the original clutch of my 1946 XB31 but it seems that the Triumph clutch is a better choice. I just stay with the original clutch.
Thank you very much for your replies. But the clutch hub is different from the one on my own B33 and M21 and can not be exchanced, this is why I ask. The gearbox mainshafts have different shapes. We would like to fit the Triumph clutch, but need trhe right hub, or a hub that will take the 6 spring clutch from a later model.
You need the hub as used on some of the pre-war gearboxes, not very easy to find but not impossible. Same hub as used on for example on the B29 and B30 clutch. Easy to recognize with it's internal thread opposite to the post war hub which has thread on the outside to pull it from the main-shaft. The difference is the location of the taper in it, very different location.
The main shaft tapper is the same on just about all BSA gearboxes.
The only thing that prevents you swapping complete clutchs is the bore on the clutch center.
Despite most of them having the same part number, some are bored deeper than others so the clutch sits further in or out.
An A 10 clutch complete will fit but you will need to put a spacer on the crankshaft to move the sprocket farther out.
You will also need to cut about 1/4" or so off the ends of the springs or they will foul on the outer chaincase cover.
The A7/A10 basket has more plates so the bolts are longer.
I ran the M20 with a 6 spring A 10 clutch for 8 years while I faffed around looking for a new original single speed chainwheel but ended up getting the two I had retoothed and going back to the single spring.
If you are prepared to do a bit of work you can squeeze a Bob Newby belt drive in. I have done it on my XB31, You have to slim the belt and front pulley down to 20mm, but it works.
