Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

That is a fine looking steam plant.

Seems you may be correct, that it is whatever the piston rings turn out to be, expanding and binding as they heat up when running.

I think you'll have to do a bit more than just pull the cylinder head to see the piston rings, as the piston needs to move beyond its TDC position and start out of the cylinder end to address the rings.

Good luck, and I hope it proves to be a simple fix!