As I remember, the Newby clutch is 68 teeth for both the single and the twin. The single clutch\belt is 30mm wide and the twin is 40mm. The engine sprocket that I use is 36T, which produces the 1.89 primary ratio. The stock primary drive ratio is 1.6, which is quite a difference. With the 36T on a twin, I would use a 22T CS sprocket and an adaptor so I could run rear sprockets in the mid 40's.
The 38T = 1.789
The 40T = 1.7
The 42T = 1.619
A 42T sprocket is pretty big. I do not know if belts are available from Bob for all these. I am sure he has them for the sprockets he sells.
I would opt for the 36T, which we know fits, and change the gearing at the back end to compensate. Of all the items that would have to be modified, it is easiest and the least obvious to change the rear sprocket.
David
The 38T = 1.789
The 40T = 1.7
The 42T = 1.619
A 42T sprocket is pretty big. I do not know if belts are available from Bob for all these. I am sure he has them for the sprockets he sells.
I would opt for the 36T, which we know fits, and change the gearing at the back end to compensate. Of all the items that would have to be modified, it is easiest and the least obvious to change the rear sprocket.
David