Piston clearance

Attached info is from my E.M. Franks Norton Motorcycles servicing guide. This covers all Norton singles from 1932 up to 1956.

My fellow riders with Norton singles such as ancient ES2 models say that, with their engines, modern technology allows them to have a rebore with just 6 thou piston clearance. Your piston size is the same as the 500cc engines.

The pre-1932 cylinder looks fairly similar in form to me when compared with the post-Edgar Franks redesign.

I'd be inclined to base clearance on piston type being used. If an ex-WD piston (they're the most common) with period oil control ring then I think that I'd go with Franks' recommendations. I've heard many stories of tightly bored Norton singles nipping up during running in.

A word with Mike Pemberton would be a good idea.

Hi Andrew,

The general situation with pistons and rebore sizes when using a piston from a reputable manufacturer is to bore to the size on top of the piston ie. 79mm + 040" exactly as the manufacturer should have ground the piston to give their recommended clearances. This may vary from one make of piston to another as the aluminium alloy might be different which could expand or distort, depending on design etc. to different degees. If you want to be safe you can always hone the bore out an extra thou. as it's better to run al little slack than too tight. Regards, Richard.