interesting possibility

While at the offerings from RGM in their belt drive department I discovered a Norton clutch center designed to fit on a Triumph mainshaft. This means it is possible to fit the Norton diaphragm clutch in a T120 or T140. That would be a huge leap forward for Triumphs making their clutch as easy to operate as a Commando. I guess you could use the modified center in a Norton clutch basket and run a triple row chain. Worth investigating.

I bought an RGM Commando clutch centre "fits Triumph" for my T140 and unfortunately it didn't fit my gearbox shaft correctly. The problem was that the clutch centre, which is a taper fit on the shaft sat too far down the shaft on mine due to the taper being incorrectly machined, and the clutch when assembled rubbed on the back of the primary chaincase, and also sat lower than the crankshaft sprocket sat without any shims fitted. I think that the clutch basket was approximately 0.090" "lower" than the crankshaft sprocket when lining up the teeth on the sprockets.

It's very unlikely that my gearbox shaft on my T140 is incorrectly machined.

If I was to do it again, I would buy one of RGM's Norton Commando clutch centres for "Triumphs" that wasn't machined and comes with just the pilot hole drilled in its centre and then get it machined correctly by an engineer correctly to fit.

Because the Norton chainwheel has less teeth on the chainwheel, (I can't just remember the difference now off the top of my head) the chain adjuster will need turning nearly to it's maximum, but if you're fitting a belt I'm not sure what you would have to do.

What about the cush?

there is no cush drive in the Norton clutch center, unfortunately. And the "cush drive" in the Norton rear wheel is wishful thinking.

exactly, which would make me think twice about the conversion on a triumph with no other provisions for one

This mod is a tad ironic in a way though as the mainshaft taper on a Triumph is a weak point, the splined Norton shaft is much stronger. So a better route is to fit a Triumph mainshaft with a Norton spline. Tony Hayward sells these (or at least he did).

I know Hayward sold an adaptor to add a Triumph clutch to a Norton shaft, I have one on my Atlas. But these are no longer available. A new Triumph mainshaft with splines would be the best way to go. The lack of a cush drive is a problem. I suspect that if the owner was a Sunday driver there would be no problem. For example, all Commandos up to 1975 had thin, hard plastic inserts and called it a cush drive. Little or no problem with gearboxes for the average rider.

I am a big fan of the Triumph cush drive, it is why I mainly kept it in the past, even on 900cc+ 8 valve racers.

But on the other hand, apart from theoretical debates, I don’t know what ‘failures’ running without a cush drive on a Triumph or Norton would cause...

broken teeth on gears, perhaps? If really pounding it with acceleration especially from a dead stop. I have read in this forum that bump starting these bikes can lead to gear damage.