I think it is a matter of torque. The gearbox and primary ratio is a torque multiplier. Nortons have a spline under the clutch, Triumphs have a taper. When a Triumph breaks a clutch key, it makes a mess of the taper on the gearbox mainshaft. The key on the Norton crank is much less likely to break then the key under a Triumph clutch. I use a Manx clutch with single row primary on my 850. I had an adaptor made which accepts Jawa speedway sprockets which float on a spline. It allows the clutch and engine sprockets to align without changing the gearbox mainshaft. The other thing is that the Norton engine sprockets with the 5 degree taper are not readily available second-hand, and those you can get usually all have different offsets. Each Norton model is different. And how many people race Nortons and change the overall gearing a lot ? The Jawa sprockets are cheap and readily available. To replace them you simply drop the chain, slip the clip off, pop the new gear on and adjust the gearbox position etc. The adaptor and puller cost about $200. I have not even hardened the spline, it was made from strong stuff. If it buggers-up, I will just have another one made.
When I change the overall gearing, I usually do it at the engine and countershaft with odd numbers of teeth on the sprockets, rather than by changing the rear sprocket. My rear sprocket is a 38 which is fairly high. I usually try to keep the primary ratio high to save the gearbox. If it spins faster, the internal loads are smaller. My countershaft sprocket is a 17 and the engine sprocket is a 24 - the clutch never gets altered, I think it's a 48. For a big circuit, I would have to go up on the countershaft.
I think 4 Jawa sprockets cost about $60 and the Manx clutch does not slip. The diaphragm clutch is probably overkill and it is too heavy anyway. A single row primary is adequate for racing as long as you wire the clip.
