Contrary to popular myth, the hole expands in a mass of metal that warms up (e.g., your sparkplug hole).
It's true. The hole doesn't expand inward (tightening it's grip) when the metal expands; the hole gets larger(!) because it's circumference increases (making the circle bigger), and therefore the length of the diameter across the hole also increases -- this will lessen the grip on the plug, assuming the plug's threads aren't cross-threaded because the stupid P.O. used an air tool to "spin" the plugs in.
If you google this concept, you'll find a brilliant essay from a Caltech professor that shows this graphically.