Wow – 0.004” clearance between the piston and valve sure sounds scary!
I didn’t realize you could run these old Norton engines that close to the “edge”; I’ve always used the thickness of a piece of corrugated cardboard as acceptable clearance. Well, it sounds like I can use thin shirt-cardboard instead and still have plenty of clearance.
You must be right. But 0.004" sounds almost sloppy for piston-to-cylinder clearance?
I would expect a good forged piston to have a low expansion rate and therefore require a tighter fit. The clearance typically varies from one piston manufacturer to another and is usually specified in the instructions with the new piston.
At any rate, I’m back to using my corrugated cardboard for piston-to-valve clearance!