The difference in thermal expansion rates between an aluminum head and cast iron block combined with the added stress caused by overheating can cause the head to warp. This, in turn, may lead to a loss of clamping force in critical areas and allow the head gasket to leak. Coolant can boil out of the radiator and be lost. Pistons swell inside their cylinders and can scuff or seize. Valve stems can swell in their guides and also scuff or seize.
Valve lifters can also stick, possibly causing a valve to remain open a little too long.
Bearings can seize. Oil thins out and is less able to protect the engine's internal components against friction and wear. Use a straight edge and feeler gauge to check the flatness of the face of the cylinder head and block deck for warpage before replacing the head gasket. Flatness can be checked by placing a straight edge on the face of the cylinder head or block, and then using a feeler gauge to check any gaps between the straight edge and head or block. The surface finish on the face of the head and block is also important.