BSA B range gearboxes

[Gavey]

Hi all,

I have a 1938 B22 Empire Star fitted with a C10 3 speed gearbox and a very odd single spring bitsa clutch that slips. I acquired a 1946 type B31 gearbox that is supposed to be a good swap for the original box but have found it impossible to locate a clutch sleeve ( 65-3930). I am now thinking of using a later B/M range (post 48 box) but believe that the mountings have to be narrowed. My question is - has anyone already done this?, and could they provide me with the measurements so that I could get the work done?. I'm over in Guernsey so don't have access to  jumbles etc. Any help would be gratefully appreciated.
Ernie Gavey

[Pete C]

Hi,

clutch sleeves for the 46-48 gearboxes are pretty much unobtainable.

I recently modified a later box to go in my 1948 B33. I'm not sure what the spacing is between the mounting plates on a C10 but I assume it is the same as for my B33, ie 2 inches, in which case you need to shave off 1/4 inch from each side of the later box's mounting lugs (2.5 inches wide).

The gearbox sprockets for the 2 types of box are also different but this should line up OK with the rear wheel sprocket using the later sprocket.
The method of primary chain adjustment is also different but I  managed to sort that using later components as well.

Using the later box also allows you a choice of clutches, single spring, 6 spring or a triumph type 4 spring. The 46-48 box will only accept the 6 spring- if you can get a clutch sleeve!

I have a few pictures of what I did if you want to contact me off line.

cheers
Pete

[Pete C]

I'm in Germmany..no boody jumbles here either! 

[Rupert]

Hi, I'm in a similar position of using an early post war B series gearbox with a pre war B series frame, no problem in doing that.
Regarding he clutch, has anybody already tried what I'm contemplating, of using the Triumph 4 spring replacement clutch with this gearbox - the mainshaft taper/keyway seem exactly the same as the M series gearbox mainshaft which the Triumph clutch is usually intended for (done it myself on an M20, it's great!).
Just hoping that may be an easy way of sidestepping the six-spring clutch nonsense and unobtainable clutch sleeve?

[Pete C]

as far as I know the 1946-48 gearbox will only accept the correct, and difficult to find, clutch sleeve for those years. THis sleeve will also only allow you to fit the 6 spring clutch.
check out the forum at http://wdbsa.nl/, you will get a definite answer from them.

[martinh10]

Rupert, you think that the taper/keyway are the same as the M series gearbox mainshaft, in which case primary chain alignment would appear to be the main issue. Do you know by how much inboard or outboard your taper is compared to the M series?
I fitted a Triumph-type clutch to my B31 M series gearbox and only to find that the clutch sprocket was 0.2" outboard of the engine sprocket (maybe I have a 1946-48 mainshaft in it - I don't know if that's possible).
If yours is the same issue let me know and I'll explain how I resolved it - I wont bore you with the details if it is not.

[Pete C]

The later box (M & B) has a longer mainshaft so you must have the correct clutch sleeve if using the earlier box and  I am pretty sure that will only allow you to fit the 6 spring clutch.
I have also fitted triumph clutches to M and B bikes and had to shim the drive sprocket to correct the alignment each time.

heres a picture of the 2 manshafts, 46-48 variant is the lower one.

[ant]

I may have a gearbox I can upload a pic at the weekend

[Pete C]

pictures of early and later gearboxes and modified later box.
Also the adjuster method, early and M20 type

[Rupert]

All interesting info to bear in mind.
Replacing my M20 clutch with the Triumph item didn't require shimming to achieve primary chain alignment but I can see that variations from one bike to another could make that necessary.
So even with a bit of shimming the basic idea is that the Triumph clutch chainwheel teeth are nominally in the same place as the chainwheel teeth on the six spring clutch they replace? That seems to work for M series and 49 on B series, so does the 46-48 B series clutch centre give a different chainline to mainshaft taper relationship, hence putting a Triumph clutch on there would put the chainline way out of place? Is that the main snag?
Good to find so much knowledge out there on this obscure topic, but I'll pencil in some time to play with some clutch bits in the shed and take some measurements...

[Gavey]

Many thanks all,

Have acquired a 49 on gearbox to modify. Just one last question Do you use the early or later adjustment setup. If the early one what size and thread of bolt should it be?  If the later M20 plate mounted adjuster system, I assume this needs to go on the front mounting bolt as there doesn't appear to be room on the back. It would be useful to know so that I can sort the bits I need while the gearbox is being modified.

Regards,

Ernie Gavey