Quote:
As the engine warms the clearance in the valves is reduced
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Actually, clearances will remain the same when engine parts are heated UNLESS the parts are made of different materials. In this case, the valve stem will expand but so does the block--that means the hole gets bigger too. In general, the hole expands at exactly the same rate as the material that was removed to make it.
If the parts are made of different materials they expand at different rates, so all bets are off. Think of the brass plug that contains the temp sensor. The brass expands faster than iron block.