Javelin,
Your concern about the bore tapers causing an engine to be tight at the top end might not be completely correct in all engines, at least for the first few seconds of a start sequence.
Pistons expand relatively more than their bores do, as they are of aluminium alloy and tend to expand relatively more than the steel cylinders.
Because of this they are made with clearances that leave them slightly SLACK in the bores when the engine is fully cold. I fully agree that if the engine is not warmed up correctly, the pistons may expand to a point where they might BECOME subsequently tight and cause scuffing, because they absorb heat relatively quickly and expand at a greater rate than do the cylinders.
But another problem is the potential lack of lubrication after an engine has stood for a while. If the engine is idled too slowly immediately after start, insufficient oil pump flow may delay lubrication to the components furthest from the oil pump, namely the camshaft and valve gear.
Years ago, I was strongly advised to kick over my high compression / racing cammed BSA M/C engine (uprated valve springs) with the ignition off until oil began to return to the tank. The drill was then to hold a forefinger over the oil return stack pipe to ensure the rocker gear was pressurised with oil immediately after start up. You really needed three legs and two pairs of hands to do this and it took quite a lot of kicking, but owners who didn't do this got cam surface and follower damage.
Best thing is to follow the engine manufacturer's guidance to the letter.
Last edited by ShyTorque; 28th Jul 2005 at 14:32.