Due to the high degree of precision and special equipment required, valve seat replacement should be left to an automotive machine shop. Seat replacement procedures vary depending on whether the engine is equipped with cast iron or aluminum heads. Iron heads use a valve seat which is originally an integral part of the head. To replace these, a bore must be cut in the head which will accept an insert. The bore must be drilled to exacting specifications for the insert to work.
Aluminum cylinder heads are already equipped with valve seat inserts. There are various methods to replace these inserts, and although most are similar, the specifications are unique to each cylinder head application. The following procedure can be construed as what is generally acceptable for aluminum cylinder heads; the actual method employed should be the decision of the machinist.
The replaceable inserts can be removed by cutting them out to within a few thousandths of their outside diameter and then collapsing the remainder. Another method sometimes used to remove aluminum head seat inserts is to heat the head to a high temperature and drive the seat out.
Upon installation, the new seat may be installed with the aluminum cylinder head heated to a high temperature, then the seat, which is at room temperature or slightly chilled, is pressed into the head. The aluminum head is then allowed to cool and as it does, it contracts and grips the seat. In certain applications, the new seat may be driven in with both the head and seat at room temperature. The calculated press-fit interference will then retain the seat in the head.