[DanEC]

Now it kind of sounds like an extreme case of too tight piston to cylinder clearances. It's hard to blelieve it would start tightening up in 4 or 5 miles but if it gets up to 100 C that fast - maybe it's possible.

Do you have the build sheet on the engine? If so, does it state the type of pistons and the piston to cylinder clearances? What compression ratio is it supposed to have?