Steel on Steel is a really bad idea! The rings only seal the gasses from bypassing the piston, not keep it centered. When the piston is going down, the angled connecting rod pushes it hard against the wall. One of the first things one learns is that you never use the same material for two sliding contact parts. Seems you are not worried about weight, so instead of aluminum you could use brass, or even bronze for the piston. Cast iron would be most people's second choice after aluminum for an I.C. engine though.Does the piston really matter? Isn't it more a concern with the rings? I thought the piston itself (at lease in an I/C engine) would not contact the cylinder walls.