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Gearbox cover thread size/form?

Working away on my gearbox.

While waiting for a couple more bits to show up, I decided to chase the cover screw threads to make sure they were clean. I have a set of taps and dies, but all are 26tpi.

The screw that hold the outer cover of my late gearbox appear to be about 20tpi.

Can anyone confirm me the thread size used? I don't want to screw things up.

Re: Gearbox cover thread size/form?

Assuming we're talking BSA....They are indeed 20 tpi...1/4" x 20 Whitworth...Ian

Re: Gearbox cover thread size/form?

Thanks Ian.

Yes BSA. 53 civilian model M20.

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