What should the margin be when grinding an exhaust valve?

The appropriate margin when grinding an exhaust valve is indeed 3/16 inch. The margin refers to the unground area of the valve face that maintains the shape and strength of the valve while ensuring it seals properly against the seat. A margin that is too small can lead to insufficient sealing and increase the chances of valve burning or deformation due to heat and pressure from combustion.

Choosing a margin of 3/16 inch strikes the right balance, providing enough thickness for structural integrity while still allowing for effective sealing against the seat. This dimension helps optimize the combustion process and maintains the overall performance of the engine.

A margin that is larger, such as 1/2 inch, may not be practical as it would reduce the effective area of the valve face contact with the seat, leading to performance issues. Too small of a margin, like 1/8 inch, may compromise the valve's ability to withstand the extreme conditions it faces, like high temperatures and pressures. Thus, maintaining a 3/16 inch margin is the industry standard for achieving the best results when grinding exhaust valves.