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Was a nice read Tom,but i assume that the rpm was low as the stand up the piston for fun :)

q Tom Booth,Was thinking for the expensive helium,you could use nitrogen cheap and plenty. For the piston rub no idea what machines you have but i would grind 3/4 or more off the bottom, away so much that only 3 or 4 small lengthwise rills stand,would reduce friction much.

I wonder if a oil can explode or even burn in a inert atmosphere that you would have in a helium pressurized stirling. No doubt there are other aspects to it.

Remember that internal combustion engines remove the carter vapors,in a enclosed stirling that is impossible so volatile gasses can explode if the compression temperature reach ignition temperature. A solution would be a "oil" that have no gasses that react without oxygen.