Hi Tatu..Glad to hear the engine is making that much torque..My 720cc 'M20' manages with the standard clutch, though I am planning to fit the 4 spring unit as the original is rather worn...
My 720cc B33, which has a lot more torque and power, already has the 4 spring clutch fitted..
The later 4 spring clutch is exactly the same clutch as a pre unit Triumph...BSA bought Triumph out in the mid 50s and immediately lifted the clutch which was superior to the 6 spring unit then in use on most BSAs...
So, it's ability to cope with power/torque is the same as it can cope with on a Triumph twin...
You will also need a BSA gearbox adapter sleeve to fit the Triumph 4 spring clutch...This is a BSA part as fitted to all pre unit swinging arm BSAs that used that clutch type...Pushrod length will also need to be adjusted...
Due to the different diameter of the gearbox main shaft taper on the M20 (which didn't use the 4 spring unit) and swinging arm model BSAs (which did) you will find that the clutch will sit approx. 6mm further outboard of the bikes centreline than previously...
This can be adjusted by increasing the width of the spacer that sits behind the crankshaft engine sprocket carrier sleeve, thus restoring alignment...
I believe you can also get modified gearbox sleeve adapters from Kidderminster Motorcycles that have the taper diameter machined differently to eradicate the problem at the outset...Personally I have always just fitted the wider spacer as described above...
Note that it is the gearbox main shaft taper diameter that differs...The taper angle itself is the same for both M20/21/B31/33 rigid/plunger gearboxes and the swinging arm boxes....Ian
email (option): ian@wright52.plus.com