[Terry Burtz, Calif]

On original engines that have had many valve jobs and rebuilds, cylinder block material was removed with each valve job and rebuild, and the valve seats have become recessed below the upper surface of the cylinder block. To compensate for this, the valves need to be shorter, and 3-angle valve seats are required to keep the seat narrow and centered on the valve face. To restore a cylinder block and replace the material removed and return the valves to seat at the top of the cylinder block, rebuilders can install valve seats or install larger valves. There is a risk of getting a press fit with the valve seats, especially if there are cracks, so many rebuilders prefer to use larger valves.