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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.
One of the best and least expensive tricks you can apply to any valve is a 30-degree back cut on the inside of the 45-degree valve seat. This creates a smoother transition between the backside of the valve and the 45-degree face, and it usually results in measurable gains in low-lift flow.