Aluminum expands a great deal more than stainless steel when heated. If you make the piston a tight enough fit to run with no rings and give good compression when its at room temperature, it may expand and seize in the cylinder very quickly when the engine heats up. The hit and miss engine I built with a stainless cylinder and an aluminum piston has a water jacket all around the cylinder, and since it is a hit an miss, it runs very cool anyways. I get away with it because my piston is a "loose" fit in the cylinder, but the viton o-ring maintains the seal to hold good compression. If the piston does expand a bit from the heat of the burning fuel acting on it, there is enough room that it doesn't seize. The piston may "grow" a bit, but the viton ring just compresses a bit more. I have ran this engine 4 hours non stop and had no problems. I do run a bit of 2 stroke oil mixed with the gasoline.