The BSA Workshop > Singles

B25 gearbox assembly

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Hi, I'm using a Haynes manual to help with the reassembly of a B25 (1968) gearbox.  However, although the text if helpful there is a lack of diagrams andpics, so any additional information would be welcome.  Thanks, Flactem.

You can see the basic layout from the parts book diagram. Just ask if you need any advice but basically most of the later BSA unit single gear boxes are similar.

Do you mean the assembly of the parts in the correct order or putting the gearbox back into the crankcase?

There are a few videos on you tube showing the technique of putting the gearbox in that I found useful.

You basically assemble everything together on the outer cover and push it all back in at once.

Remember there is a spring attached inside the crankcase that you need to push against. The knack is knowing if you just need to push harder or if something isn't lining up correctly.

It took me about 20 mins to get it right.

As said, it's a lot easier to assemble everything on the inner timing cover first and then slide it into place. That way you can check everything is properly located especially the selector forks. Make sure you include the two thrust washers/shims - nos 94 and 100 in the parts diagram. Once assembled, it's best to check you can select all the gears.

My advice would be to junk the Haynes Manual as for the unit singles it is quite frankly pathetic!
Get yourself a copy of Rupert Ratio Unit singles manual vol 1 the engine which are back in print now:-

Vol 1 23.95 inc uk postage
Vol 2 25.95 inc uk postage
Vol 3 back in print soon...
All available direct from Ratio Publishing.
Authors signed copies only available from
or email

They cover everything you will need to know. No rebuilder of unit singles should be without one!


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