Hello Tony the shaft size should be either 5/8 or 3/4 depending on the clutch size which should be the same. best to use a vernier caliber to make sure though.
Other people on here probably know more about it who have more experience than me.I replaced the engine on my model 33 with a briggs and stratten engine i was able to get a 3/4 shaft size and got a clutch to suit .it I hope that this helps
Cheers Shaun
