We all know that the valves have angles on the faces that mate up to the valve seats, and that the valve seats have the "same" angles. Even though these angles are the same the valves may not match exactly the angles on the seat, and if the valves and seats are not matched gasses can escape under compression because of a faulty seal between valve and seat.
We need to lap the valves so that all the faces match and seat correctly. This is achieved by using a valve grinding paste, and rotating the valves back and forth so that the valves are lapped with the seats, this means that the valves and seats form a good seal and gasses will not escape until the valves open.
Cruser
