I found this article ! "New seats may also be required if a cylinder head has been straightened or welded, if there’s any evidence of corrosion around the outside diameter of a valve seat, or if the engine is being converted to run on a dry fuel such as propane (LPG) or natural gas." "Some experts say when late model aluminum heads are reconditioned the valve seats should always be replaced to maintain correct valvetrain geometry. This applies to overhead cam engines as well as pushrod engines. It’s expensive but is usually necessary to restore proper installed valve height and valvetrain geometry. " "When seats get too hot, microwelding occurs between the valves and seats. The valves are harder than the seats so microscopic particles of metal from the seats stick to the valves," says Emert. "When the engine cools, these particles are then washed into the exhaust. This causes rapid recession of the seats and is most common in dry fuel (LPG or natural gas) engines."
