B31 gearbox

[Ronnierockets]

Hi I'm new to the forum ,and this is my first post.
I have a '48 B31 rigid basket case ,and would like to fit the later type gearbox .
The early gearbox mounts are 2" and later mounts are 2 1/2"
I have thought of using the straight type gearbox / engine plates,as the early type are off set to suit 2" for the gearbox and 2 1/2"for the engine.
I believe this change was done on later rigid models
Has anyone here any experience on this?
Thanks..Ronnierockets....

[Keith L]

Hi. I have known the gearbox mounts on later boxes to be cut down to fit the existing mounts. I don't recommend it but it is possible with careful thought on alignment of sprockets etc. When I bought my 1946 B31 it contained such a box but I have reverted to the standard box and recently sold the modified one.


Keith L

[Ronnierockets]

Hi Keith, thank for your reply.
Was the same amount removed from each side of the gearbox? (1/4")
Thanks..Ronnierockets..

[Keith L]

No, it was all taken off one side. I think it was the left hand side (Drive Side) but I can't be sure. Keith

[Ronnierockets]

Thanks keith,
I will check that out when I fit the engine bottom end in the frame,
and rear wheel hub and sprocket,will have to get a rear wheel axle and fittings first,as I don't have these.
Many thanks ..Ronnierockets..

[Pete C]

I was thinking about fitting a later gearbox to my 1948 B33 bike which has the 46-48 gearbox and needed a lot of work doing. If I remember rightly, when I took measurements from the two types of boxes I reckoned I needed to take material of both sides of the lower boss to maintain the alignment of the sprockets so I don't think just removing material from one side is optimal. I may be wrong as I managed to find parts and recon the original box so didn't go ahead with converting a later B box to fit.

There is also the question of primary chain tension. The method of moving the gearboxes fore and aft is different so you would either need to modify the gearbox shell to accept the adjuster screw or the frame to accommodate the gearbox adjuster set-up for the M and later B models.

I have to admit that I found the 46-48 box a pain to work on and if it gives any more trouble I will again seriously consider the conversion as the later boxes are a delight in comparison! (if anyone does know exactly how much material should be removed, and from where, to make a later box fit, I would welcome a second opinion!!)

Good luck!
Cheers
Pete

[cdsdorset]

I looked into this conversion. I made some mounting plates so the later box could be just bolted in. The problem arose with the lack of room to fit the primary chain adjuster. I ended up putting the original type box in. Maybe later I will try to overcome the problems and see if it works.

[Ronnierockets]

Hi lads, thanks for your reply,
I have heard that the earlier 'box isn't as beefy as the later one ,and that's one of the reasons that I want to use the later type,the other reason is that I have the later box only. ( spare for my m 21)
As for the primary chain adjustment,I was thinking of an adjuster in the primary similar to a 10 plunger,
However it's all a bit up in the air at the moment, as I'm only sorting through what I have,and what I need,  and all your Imput  is very welcome.
Thanks..Ronnierockets..

[Ronnierockets]

I have found a b31 gearbox 1946 to 1948 type to suit my needs,
After inspecting the internals,I find that I need the layshaft bushes ,drive side bearing ,sprocket,
And cork oil seal,
Does any one know who can supply these items?  Are they hard to come by?  as I believe these 'boxes
We're used for the early b31 for two years only.
Thanks..Ronnierockets..

[AdrianW]

Although it was some years ago (and for the later gearbox) I found C & D Autos (BSA Spares) to be very good, supplied everything I needed. Give them a call...

Cheers,
Adrian
Bristol UK

[Mark N]

Just had my one rebuilt by someone i know. He struggled to find a couple of bits, but did eventually succeed.
He has done a modification that incorporates an oil seal rather than the original idea of an oil-return spiral thingy.
One thing to watch; remove the speedo drive before forcing the case open. A previous owner had badly damaged the speedo drive on mine.
Sorry i can't be more technical 

[Ronnierockets]

Thanks Adrian and Mark,
I have located the bushes on eBay
But finding it hard to locate the gearbox sprocket, can any forum member help?
Thanks...Ronnierockets..