Hello I live &work in Thailand . I have bought a rusty heap of bits which eventually should result in a nice, albeit not quite totally original BSA M20 .
Current struggle is the gearbox . This came disassembled. I have cleaned the parts, renewed all the bushings & bearings and re assembled it . I have about 10 mm play on the main shaft.I can pull the shaft to the left (drive side ) for about 10 mm , the smaller of the two kick starter springs then pulls it back when I let the shaft go . This doesn't seem quite right or is it ?
I have the BSA M20 maintenance and repair manual but it is not exactly helpful. A clear exploded view of the gearbox would be.I may well have omitted washers ,spacers etc .
The manual states that it is important that the gearbox assembly is put together witht pinions in top gear position but what the top gear position is is not exactly clear for the drawings . Any tips ?
Many thanks in advance
Maxim Jurgens
Phuket , Thailand
Hi Maxim, welcome to the group.
There is definitely something wrong with 10mm of play.
There should be .005"
Is your gearbox an early or late one, the early ones had the clutch operating arm operating in the near vertical plane where the later one is horizontal. if it's the later one the main shaft bearing on the kick start side has a retaining circlip, if this is missing and the bearing is a loose fit it might cause this problem. On second thought if it's an early box and this bearing is loose (it has no circlip) it may be the cause as well.
Anyhow check an exploded diagram of your gearbox and make sure everything is in place.
Hans, thanks for the drawing.It is extremely helpful,just what i was looking for !
Play on the mainshaft is not 10 mm, after measuring it turned out to be 4 mm. Sorry about that.The only spacer on the main shaft which I have is 2.98 mm /0.119 in thick and it lives between the rightmost pinion and the ball bearing in the inner cover. Hans' drawing shows two additional washers,would you know the dimensions of these? If I line up the pinions in the box as per the manual the mainshaft has 2 mm play to the left and 2mm to the right.Play to the right (kickstarter side) I can eliminate with shims but other then placing shims between the clutch and the gearbox sprocket I can't see where I can place shims to eliminate play to the left .
Any tips ? Cheers from sunny Thailand
Hi Maxim,
OK, 4mm will be a lot easier to deal with. It sounds like you have item 49. Item 48 is an oil retaining washer that sits under the bearing in the inner cover, item 50 is an oil flinger washer. Both these measure 0.78mm in thickness so there's 1.56mm. Check that you have washer 56 between the bearing 55 and the sleeve gear 57. Extra steel shims can be placed between 55 and 56 and between 49 and 50. Lastly the inner cover gasket needs to be of the thin type, 0.4mm I think when dry.
Let us know how you get on.
