The 1957-on Norton gearbox was reportedly good for up to 50 BHP.
All that I've seen that came from 750 and 850 Nortons, which probably
made more power than that, had noticeable pitting of the gear tooth
flanks. They seemed able to serve OK whilst in that condition, but that
could be because I was poor, the bikes were old, and didn't get thrashed
so much.

Jordan

John Martin wrote:
The gearbox can take
anything the engine can throw at it