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The main reason for the piston to head clearance is to compensate for rod stretch, piston pins bending, crankshafts bending and piston rock. The higher the rpm, the more this happens, which is why piston to head clearance has to take rpm into account. As for engines growing due to heat expansion, I have heard many anecdotes of how much engines grow. In reality, if an engine were to grow as much as some people claim, then the bearings would be falling out of those engines.
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