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Besides guide wear, it's also a good idea to check the valve face margin. The margin (arrow) is the portion of the valve between the face and the outside diameter of the sealing face. A wide margin is essential especially for exhaust valves. Valve margins often become very thin if subjected to excessive valve face grinding. A thin exhaust valve margin not only decreases flow but can also overheat, which can contribute to detonation or pre-ignition problems.