During cylinder head machining how should a valve seat that is measured wider than specifications be repaired?

• A. Throating, then a top grind on the seat
• B. Fresh grind of the original seat
• C. Grind 2 degrees less then specification
• D. Grind 2 degrees greater then specification

Answer: (A)

When a valve seat is wider than spec, the aim is to restore the correct seat width without upsetting the seat’s geometry or alignment. The proper approach is to narrow the seat first and then finish the surface.

Throating removes material from the inner portion of the seat, effectively reducing the overall width while preserving the outer edge and the seat’s angle. This step corrects the excess width without changing how the seat contacts the valve face.

After the seat has been narrowed, a top grind finishes the upper seating surface. This ensures the seat is square and true to the valve face, providing a proper, uniform sealing surface and the correct overall height. Doing only a top grind or trying to change the seat angle would either fail to address the width properly or disrupt the valve’s sealing geometry, leading to poor seating or misalignment.

So, narrowing with throating first, then finishing with a top grind, is the correct sequence to repair an oversized valve seat.