After repeated closing against its “seat”, wear appears on the valve's face as a ridge. This is called valve recession, and it gradually impairs the valve’s sealing. The valve seat wears too of course, by widening and cratering. And valve guides wear and occasionally must be replaced. But most of Honda cylinder head rehabilitation focuses on the valve seats.

The valve seat in the head has three angles. The sealing surface, where the valve will reside, is a 45 degree angle. This is the working part of the seat. Valve seats also have two additional, adjacent angles: a 60 degree surface transitioning from port to seat, and a 30 degree one transitioning from seat to combustion chamber. These are sort of oversize chamfers that ease airflow across the valve seat. So critical are these angles that the Superflow corporation, makers of the most widely-used cylinder head airflow measuring equipment in the industry, holds that the valve seat has the most influence on cylinder head flow performance.

The first step in a valve job is to lightly machine the seat’s 45 degree surface to square it up relative to the valve guide and remove pitting. Then Prussian Blue is carefully smeared on the seat, and the valve pressed against it then removed to “register” where on the valve’s face the seat is contacting it. This contact must be centered on the valve’s face. If it is off-center, then the seat’s supplementary 30 and 60 degree angles are machined to move the contact where it belongs. If for example the contact is off-center toward the combustion chamber, then the 30 degree angle is machined slightly to move the contact point in the other direction. If on the other hand the contact zone is off-center in the opposite direction—toward the valve guide—then the seat’s 60 degree angle is machined a bit to move the contact appropriately. Once the seat is centered, now that 45 degree contact surface needs to be touched up again, smoothed and widened or narrowed as needed for good sealing and long life. The 30 and 60 cutters or stones are used in equal amounts to adjust seat width.

Then there is valve lapping. Valve lapping is of course advocated by most manufacturers today, but it's goal is to compensate for improper valve seat machining or even no machining at all. Valve lapping is not part of a valve job or a substitute for a valve job. Sure, bike owners just want to improve valve sealing and not invest in expensive tools. But where lapping is concerned that’s wishful thinking. On a Honda, the whole reason for valve work in the first place is valve recession. When a valve is lapped, what do you think happens to that recession? It potentially increases, of course; it gets worse. It sure isn't removed. Thus absolutely nothing is solved.

Granted, valve seat machining tooling is relatively expensive and not likely to be found in the DIY toolbox. It is best to find a machine shop you can trust. But it is not completely out of the realm of DIY providing you can and want to invest in the tools. Very simple hand tools have been endorsed by and at times retailed by the Big Five. The Neway valve seat cutter is probably the best-known. This carbide rasp cutter costs about $100 per angle, resulting in about a $300 investment per valve size, so $600 for both intake and exhaust in cases where the same cutter can’t be adjusted to both valve sizes, which in some cases it can. Add to that the necessary pilot(s) and driver and you’ve spent a between $600 and $700. But the Neway system is far from the best.

Compression Chronicles part 4