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You can lap the face and seat, but that's when both parts are at room temp. What happens when the combustion chamber is supporting exhaust temps of 800ºF? The valve expands a little in size, so does he valve seat, and the room temp contact surfaces no longer coincide.
In addition, most valve face/valve seat angles are ground to provide a 1º interference sealing surface - it creates a point contact. Grinding an impression of this interference point into the face of the valve doesn't make sense.
I vote for Prussian blue to indicate the location of the interference seal on the valve face.
Sherwood
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