Asking for advise on lapping/polishing

You not only need a smooth surface, you need a consistent diameter. The easiest way to achieve that is with a adjustable lap. But you are using bronze and most commercial laps are brass. The lap needs to be a softer material than the cylinder. That leaves aluminum or lead as readily available materials. When I built my Sterling, which has a cast iron cylinder, I purchased an adjustable barrel lap from MSC. It worked very well and my cylinder finished nicely. See

. You could make a similar lap out of aluminum. A drill press or lathe will provide the driving force.

BTW, graphite makes a superior piston.

Randy

Aluminum or even hard wood.