Usually the plug washers are 0.8mm thick or 1/32 of an inch. I did not see a thickness for the washers for spark plugs, but they will work for glow plugs. An interesting thing about the 1/4 – 32 glow plugs, is of the plug body enters into the chamber a little bit, a better performance has often been noticed. It is not from the small gain in compression ratio, but to to the gain of how the fuel is ignited and burnt. The same is being investigated with spark engines too.