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Re: BSA M20 Gearbox tight

Hello all,

So I fitted all the gears when rebuilding my M20 gearbox. Checked and fitted fine, turned and selected the gear ratio lovely.

Inner cover on, then there's the pain. The gear selection system, followed the manual and fitted the springs, bits and bobs, peg sits well, bush, then change pedal to check I could get my gear selection, and it's jammed tight! I have no movement in any of it at all. Checked, stripped again, solid! Take it off, check the gearing in the box, sweet.... but no movement at all with my selector.

So, my problem is in the arm, or somewhere on that outer side of the inner gearbox cover. Any advice would be appreciated indeed!!



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Re: BSA M20 Gearbox tight

Do you have the shafts and shifting forks shimmed correctly? Also are you fitting a gasket to the inner and outer covers?

Re: BSA M20 Gearbox tight

Hi Danny
I have been through this pain very recently - now I have a box that internally is precise - still some work to do on the external change mechanism but at least I know if it’s an issue - the box can remain in the frame
Shim if required & check each shaft separately ensuring you use a correct inner gasket ( thickness matters here) & secure the case with the 2 screws to the rear & 2 bolts to the front to check for binding & / or tightness
Once this is done - what I did was check all shafts - no gears installed - for movement & final lateral play
Assuming that’s ok - reassemble with all gears in place & check for gear selection after you have “timed” the quadrant with the control shaft & put it together in top gear
Make sure you have plenty of biscuits & access to hot water for tea or coffee - it’s far from a 5 minute job

Personally - I learned a lot from my gearbox build experience & despite the issues I had - I am far more confident now in tackling the next one which is awaiting me

email (option): Jonnyob1

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