Gearbox inspection cap breather

I have just discovered that the aftermarket (vintage rebuilds) clutch inspection cover in the top of the gearbox has no breather hole. How critical is this? Does the gearbox build up pressure? Should I drill a hole in the cover and if so what diameter?

edward said:

I have just discovered that the aftermarket (vintage rebuilds) clutch inspection cover in the top of the gearbox has no breather hole. How critical is this?

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Not exactly critical.

edward said:

Does the gearbox build up pressure?

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No.

edward said:

Should I drill a hole in the cover and if so what diameter?

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That's up to you, here's a previous discussion on the subject: https://www.accessnorton.com/NortonCommando/gearbox-breathers-whos-got-one.18115/

And another: https://www.accessnorton.com/NortonCommando/850-inner-outer-gearbox-leak-question.11361/

Wouldn't the hole the cable feeds through service the same purpose?