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My hot side cylinder is 3" thick walled aluminum tube. The heat pipe/collector connected above this is steel. Originally planned on casting some epoxy pistons, but after testing out solar fresnals and parabolic mirrors I relised I need something more robust. The resin I have deforms at 230F.

I'm sure I can get some slugs machined to the diameter of the pipe, but how would I seal them? I want to stay away from anything that needs a lubricating system.

Appreciate any ideas.

May have answered my own question. I'm going to see how a few coats of ceramic paint will hold up.

Still open to ideas

Wouldn't a graphite piston work well?