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.