Cylinder/Piston clearance ?

I'm making Kennion's 'Tina' mill-engine. I've completed the cylinder (gunmetal, 1" bore) and the piston (gunmetal, 1" diameter). The piston is 3/8" wide, with a 1/8" groove for graphited yarn packing. I'll initially run this engine on air, but may want to try it on steam in the future.

What clearance should I allow between the piston and the cylinder ?

I don't really measure the actual size but expect on a cylinder that size it is 1thou or less. I just aim for a free moving piston that will take some time to drop under it's own weight without packing when one end of the cylinder is covered.

Thanks. I think I have the right 'fit' - with both cylinder-end covers fitted it's much harder to move the piston if I block the passage-ends on the valve-face with my fingers. Does the same clearance apply when running with steam rather than air ?

You might want a little extra clearance when running on steam to allow for thermal expansion of the aluminium (?) piston being possibly a bit greater than the cylinder but it's probably not critical.

You have to remember it is not the piston that provides the seal, but the piston ring / packing. So a bit of extra clearance on the piston ensures it will not bind but does not mean it will leak by to any problematic extent. Same on the piston rod gland seal.

Piston to bore fit is more critical on Stirling engines that don't have piston rings and on high performance model aero engines and the like. But not so much on low revving steam engines.

Thanks. It was a silly question in a way - I've built a Lady Stephanie (see my profile pic) that runs nicely on air or steam, but that was 25 years ago so I can't remember what clearance I gave it or why

Yes none of us are as smart as we were 25 or more years ago! Welcome back to the hobby. The Lady S looks nice.