[yorkie_chris]

Exactly. Exhaust valves especially, as you've got very hot gas going past the valves every time they open. Then the very hot exhaust valve slams into its seat a few hundred times a second so gradually the seat and valve wear together taking up that clearance.

When the clearance gets to zero you get a big problem, the exhaust valve can only cool itself by heat conduction from the valve to the seat. So if they never touch the valve rapidly heats up. Cheap way is the valve then burns out. Expensive way is the valve starts to act as a hot-spot and initiates combustion instead of the spark plug causing preignition which can hole pistons.