1946 B31 gearbox woes

[Ade Overs]

I've just rebuilt a 1946 B31. Unfortunately it has the early gearbox (with the speedo drive exiting from underneath the drive side) Spares for this box seem pretty rare. One problem I have is the kickstart jams occasionally, presumably due to the rather worn ratchet pinion (pt no. 65-3283) I can't find one of these anywhere. can anybody help?

having this early box also means that I can't upgrade the clutch to a later 4 spring type, due to the fact that the mainshaft has a different position and angle of taper compared to later boxes, and the supplier of these clutches (LF Harris?) don't seem to make an adaptor centre to fit the early '46 to '48 boxes. My question is; would it be a good idea to fit a slightly later rigid/plunger box which would allow upgrading to a 4 spring clutch and better spares availability? In so doing, would my current in-tank speedo work ok from the different take-off from the top timing side of the gearbox?

Any other suggestions to solve my current woes would be gratefully received!   

[Pete C]

there have ben a few threads on this subject already. The early boxes are a bugger to find parts for 
have a look at this thread and see if it helps
https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=12813.msg69047#msg69047

good luck!

[Dean Southall]

Have you tried Kidderminster Motorcycles for the clutch adapter?

[Pete C]

the adaptors available from Kiddiminster and others are not suitable for the earlier box. You are stuck with the 6 spring with the 46-48 box.

[Derek996]

I was at the Belgian Classic TT races in Gedinne a few years ago and there were a few rigid framed BSA racing, all highly modified. The one in this (not very good) photo clearly has a swinging arm gearbox, presumably using a set of special engine plates have been made to fit it. The bike was listed as a ZB33 but has an alloy top end, girder forks but an 8" front brake.
If you have the resources anything can be done.

[Ade Overs]

Thanks for your help guys. I think I'll pursue the later box idea. Still not sure about speedo operation, though. Any thoughts from the people who have done this gearbox mod?

[idie]

My 1949 M33 has the slightly later gearbox then your B31 but I have fitted a 4 spring clutch to it I bought from Kidderminster. The problem with fitting the later box is that the mounting lug is I believe wider so you need to trim it to fit the early B31. I read about this somewhere about how much to take of. I can't remember where unfortunately.

[Derek996]

It's all here - https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=12813.msg68583#msg68583

[idie]

Thanks. I knew I had seen it somewhere.

[Mr G]

Hi, same here. I have spent considerable time rebuilding a 1946 box but have ended up with a primary chain misalignment problem. The box was 'got at' by previous unknown people who filed selectors, used normal washers for spacers etc. to get some gears working. Shockingly bad work I may add. All new phosphor bronze bushings made and fitted and damaged gears repaired. All gears now selecting perfectly and thrust washers set up to center the gear selectors when not engaged. As I had to guesstimate some spacers widths, I am now thinking that the spacer/thrust washer between the sleeve gear and main shaft end is way to thick. This would shorten the extended main shaft length which is my problem. Or a wrong clutch adaptor is fitted (more than likely). I thing this is what is wrong as if I remove the thrust washer between the sleeve gear and main shaft end the selector dog gear (dog clutch) will rattle on third gear when in first or second and near impossible to select third. So, can someone with an assembled box measure the length of the shaft protruding out of the sleeve gear for me. And maybe if a more knowledgeable person can advise as to the actual thrust washer positions and thickness of same. Parts books and other illustrations for this box are very confusing as they cover many models and not very clear.

Thanks in advance, Gid

[Mr G]

As an update to my query, main shaft in my gearbox protrudes approx 1.98" or 50.2mm from sleeve gear.

Gid

[Mr G]

Here is some info regarding 6 spring clutches that I have found during my woes with my '46 gearbox. The most important part is the back plate. The problem is all the aftermarket plates are made from hot rolled steel (at least the ones I purchased from UK suppliers) that flexes under the normal clutch spring pressure. The originals are made from cold rolled steel, much stiffer. They also measure thinner approx 2.4mm against original 3.2mm. When clutch lever is pulled, the back plate will try and return to its normal shape which will push the clutch basket out against the pressure plate which in turn causes severe clutch drag as clutch plates will not fully separate. Proved by using a NOS back plate and digital dial measurements. Also, the aftermarket steel plates are also made from hot rolled steel which I have also purchased from different UK suppliers and all have arrived warped. These also will cause clutch drag when warped. Impossible to straighten. It beggars belief that whoever is making these plates are getting away with s**t quality! I was very lucky in finding NOS parts which I must say solved my 6 spring clutch problems. I can hand spin the center hub now when pulling the clutch lever. No more hard gear changes etc.. Now I must contact the companies I purchased these shoddy parts from and get a refund.