If a valve seat is damaged, cracked, loose, receded or too badly worn to be recut or reground, it will need to be replaced. Not doing so can cause a variety of problems: loss of compression, valve burning, valve failure, valvetrain wear and breakage, even head and valve damage if the seat comes loose.

Another reason to replace a seat is if a valve has broken because the seat is not concentric with the guide. Misalignment between the seat and guide causes the valve stem to flex every time the valve closes. When this happens, the counterbore must be remachined if possible to realign the seat with the guide. New seats may also be required if the engine is being converted to run on a dry fuel such as propane (LPG) or natural gas.