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Valve Jobs Done Right. A close-up of a Honda CB750 SOHC valve seat. Though the upper 30 degree and the lower 60 degree shapes have been rough cut using Neway cutters, the 45 degree cut in the middle, the one that counts because it is the actual sealing surface, has the benefit of a stoned surface, the best possible with hand tools. Ah, valve jobs.
The valves are chucked up in the valve grinder and ground at 45 degrees. A typical valve will require 2 or 3 passes to clean up the face. Each pass will shave only 0.0005â or so from the valve. Taking light cuts allows the minimum amount of material to be removed to restore a square face. As more of the valve is ground (or worn) away, the valve will sit deeper into the seat. This reduces compression, but more importantly, it also changes the flow characteristics of the valve curtain. It never changes it for the better, either.