Replacing the Valve Seats of a Race Engine
So people say, why should you worry about the valve seats on your trusty but rusty old motorcycle? There is up to 4 to 5 horsepower difference between a worn cylinder head and a rebuilt head. The rebuilt head makes more horsepower because it is refurbished with valve seats that improve durability and provide a better compression seal.Since valve wear is mostly caused by contamination, the cleaner your air filter, the longer your valves will last. On a well-maintained stock motorcycle, the valves should last a long time. But every time the valves are replaced, the head needs to be replaced or rebuilt.
Valve-seat wear is most noticeable on the valve head itself. The valve's contact face gets cupped where it mates against the valve seat. When the valve is cupped, it changes the dynamics of the surface fit against the valve seat. The more the valve cups, the quicker the valve seat wears, and the more dramatic the power loss.
It is important to note that the contact face of the valve never touches the aluminum casting of the cylinder head (in case of an aluminum cylinder head). Instead, it closes against a composite iron ring that is countersunk into the combustion dome of the cylinder head. The valve ring is about 5mm thick, which means that is is thick enough to be recut. Every time a valve job is done using the stock iron seats, it will increase valve wear because of work hardening of the metal ring.
The stock valve seats can be replaced with aftermarket copper-style seats – the most common being Ampco 45s and copper/beryllium. But that is mostly done on high performance race engines. Also, you might think that these exotic valve seats are harder than the stock seats, but just the opposite is true. The copper-based materials are softer than the stock iron valve seat. Beryllium is used in the alloy to allow the valves to shed heat faster and seat quicker. Another thing is that copper-based valve seats need to replaced much quicker...
Heat transfer is very important in the combustion chamber, and copper-based metals can wick heat away better than iron. Wicking the heat away from the valves helps the engine retain horsepower.
Engine make their maximum peak horsepower after just a few minutes of running, and then the power tapers off. All things being equal, the copper-based material won't make the horsepower go up, but it will make it drop less over the valve's lifespan...
Replacing the stock valve seats with copper/beryllium seats isn't cheap. In fact, it often costs about the same to upgrade copper valve seats as to buy a brand-new cylinder head. However, the benefits of replacing the old valve seats instead of buying a new cylinder head are that you don't have to redo any other modifications on the head.
Please not forget that copper-valve seats are only a serious option for a dedicated race engine...
