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I need to think about the remaining ali bits as I am not convinced that Ali makes a good bearing surface for the Gudgeon pin or the crankshaft. I'm off to look at that original engine Jo
I saw your comments above for the fit of the piston, and by far cruder methods than yours have managed to get it so that it drops under its own weight with a dry piston and cylinder, but just slides down slowly when oiled. It took me several hours to take off the last half thou or so to convert a stiff push fit into a free sliding fit!
I was disappointed to find that if I blocked the cylinder outlet at the governor end, the piston didn't stay put with the air cushion under it. Then I realised I'd drilled and tapped the piston head for the gudgeon housing---.