The title line pretty well says it all. I need to get a 3/8 piston (bronze) to fit a 3/8 bore (bronze) with an air-tight fit while still allowing rotation of the piston. I've reamed the bore to a smooth finish but don't know how to proceed. O-rings are out of the question as the movement of the piston is rotary covering/exposing a length-wise slit port. (Corliss valves).