Author Topic: B21 gearbox gaskets  (Read 548 times)

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Greenfield

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B21 gearbox gaskets
« on: 30 April, 2024, 16:26:39 »
Its slow progress with my B21 restoration. The more I uncover the more I find dodgy work. So what's new. I'm not aiming for a show bike but need it to work and get me from A to B.
As the gaskets for the covers of the gearbox are no longer available, can anyone suggest the thickness of the material I should use to make my own?

Steve.S

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Re: B21 gearbox gaskets
« Reply #1 on: 04 May, 2024, 16:42:08 »
I would think about 0.010" to 0.015" thick. But remember that the thickness will affect layshaft and mainshaft endfloat. If excessive, this will result in a poor gearchange.

Group Leader

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Re: B21 gearbox gaskets
« Reply #2 on: 16 May, 2024, 20:39:31 »
That's topical, I cut a new one out for my B21 only yesterday!

First time I've been in the (any) gearbox, necessitated by the one piece kick-start return spring becoming the two part version.

It's a game getting it all back in and working isn't it.    How does one correctly pre-load the spring and for that matter, check the end float of the main and lay-shafts?

Alan
« Last Edit: 16 May, 2024, 21:30:56 by Group Leader »

militaryron

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Re: B21 gearbox gaskets
« Reply #3 on: 17 May, 2024, 05:12:31 »
I don't know what the spec is for a B21? But I've rebuilt M20 and B30 gearboxes. I assemble the gearbox with just each shaft in turn and no gasket and try to achieve no end float. On final assembly with the original 007" gasket, will give the required end float. I'm happy with 005"-010".

Ron

Steve.S

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Re: B21 gearbox gaskets
« Reply #4 on: 18 May, 2024, 18:14:14 »
Alan,
I don't know much about these modern gearboxes with internal kickstart springs, but I wonder if you could loop  piece of string around the spring hook and pull it tight to pre load it? You just judge the pre load by how it feels.
Regarding endfloat, assuming the gearbox was working OK before the spring broke, I wouldn't worry about it too much.
However, I would make the gasket about the same thickness as the old one. When it's all assembled, grab the mainshaft and push it in and out. If you have a DTI gauge, use that, turning the shaft through 360 degrees. (Endfloat will alter if something is bent). Otherwise just judge it by eye.
I don't think this need concern you at this stage, but it might be worth mentioning that endfloat would normally be accurately checked with the component clean, dry and free of oil.
Finally, if you can't select gears after you've reassembled it, it probably means the indexing of the teeth on the selector shaft is incorrect.
Best of luck, and let us know how you get on.

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Re: B21 gearbox gaskets
« Reply #5 on: 02 June, 2024, 23:01:54 »
Thanks Steve.       

We managed it in the end with a blade in the screw driver slot and some Molegrips.    I did need two pairs of hands; one pair to put the pre-load on the other pair to fiddle the selector shaft and layshaft bearings into position without disturbing the selectors.   After a couple of goes we managed to find all four gears and a neutral (although I suspect there maybe a few other neutrals lurking in there also - there were before  :) )

Alan