Layshaft 'o' ring size and material?

What size is the layshaft 'o' ring?
I have measured 22mm internal diameter by 2mm cross section from the current one, but the cutout in the layshaft is more like 2.5mm.
Also, is Nitrile rubber OK, or should I go for Viton?
Cheers
AlG
I have measured 22mm internal diameter by 2mm cross section from the current one, but the cutout in the layshaft is more like 2.5mm.
Also, is Nitrile rubber OK, or should I go for Viton?
Cheers
AlG