I am thinking of modifying a (“standard”) rigid/plunger type gearbox to replace the 1946-48 type fitted to my 1947 B33. The original gearbox is OK and I have refurbished it but I would prefer the more robust version, which are easier to come by, if I can work out how to modify the case and organize a way of tensioning the primary chain.

Has anyone done this or can someone offer advice, tips etc.

First problem is how much and where to take material off the casing so that it fits into the narrower frame plates (2 inch instead of 2.5 inch wide) and lines up with the rear wheel sprocket.

Measuring the distance from the sprocket to the boss on both types of boxes suggests that the standard box would be around 6.5 mm too far inboard if the driveside end of the gearbox locating boss (is that the correct term? I’m a biologist not an engineer) is left intact.

Standard gearbox


1946-48 gearbox


A gearbox was recently offered for sale which seemed to have been modified, perhaps for this purpose but to my mind would not fit as only material from the timing side has been removed.


What I reckon, and please do correct me!!, is that I should take 6.35mm (1/4 inch) off each end of the boss so that it fits between the plates. It looks to me like that is maybe what BSA did and then made up the difference from the driveside main bearing to the chain line by extending the spigot on the sprocket.

Standard


1964-48


Has anyone solved the problem of chain adjustment?



All and any suggestions very welcome (note: measurements taken in a freezing shed so not accurate to the last thou!)

cheers
Pete :smiley:

Pete before I scrolled down to your possible solution comment, I had also worked out from your first two pictures that 1/4" off each side of the lug would line the later sprocket up and fit between the plates. As for chain adjustment, It looks like the M20 system could be adopted.

Although to my mind it's a stupid system......As is the rear chain adjustment.:rage: Why they didn't just use the tried and tested simple methods adopted by other manufacturers is a mystery to me. Ron

email (option): ronpier@talk21.com

Hi Ron

Thanks.

The B33 frame has a bolt going through the plates where the forked part of the M20 adjuster goes so I will probably get away with just extending the adjuster to replace that bolt. Although I think some of these adjusters are already in that form (65-4596), if I can get hold of one.

M20


B33

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The early B33 inherited that gearbox adjustment from the B30 and a few pre war 'B' models..Ian

email (option): ian@wright52.plus.com

Hi Pete, We have an 1947 B33 which also had a later box fitted with the same one side cut out as your picture. Have not checked if that cutout was correct regarding the chainline but replaced it with a correct early box.

They are nice little lightweight bikes but I agree that the early box is probably to light for the 500.

For me, cutting down the feet of the case would be out of the question.....you also have to remove the ribs which supports the feet. If I really wanted it I would first concentrate on modifying the plates in a way so they can take the later box.

Good luck, Michiel

I've got plenty of spare gearbox main cases if you want to experiment...Ian

email (option): ian@wright52.plus.com

thanks Ian, I think I have a spare case in the parts bin but I'll keep the offer in mind if I can't find it. I hope to get to Torrington as soon as this virus allows!

I am a bit concerned about the strength of the case if the webs are cut off but thought I'd go that route before making new plates. What's the worst that can happen? :thinking_face: :fearful: :scream:

Hi
i have this housing gearbox NOS
if u interested i can send photo to u very soon
cheers

email (option): sl_hussein@yahoo.com

Hi Hussein,
thanks, but I'm sure I have a spare servicable case that I can use.:+1:

Hi Pete
i hope good luck to u
cheers
Hussein

email (option): sl_hussein@yahoo.com

Morning Ian, email sent regarding your gearbox casings.

Many thanks.
Martyn

email (option): Gwyndwr@icloud.com