I am researching the design of engines and I noticed that most pistons have piston rings. My understanding is that the piston ring helps create a gas-tight seal.
Is the piston diameter machined to exactly match the cylinder diameter? Or is the piston diameter undersized, and then the rings make up the slack and exactly match the cylinder diameter?
I am asking because I would like to try machining a crude piston/cylinder arrangement. My idea would be to machine some round stock steel and purchase piston rings off the internet.