I seem to remember some engines using a 1/2" pipe thread on the plugs, with no gaskets. Maybe I'm thinking of some of the farm tractors from the 30's and 40's. I had a couple of the old take-apart plugs with the removable porcelain in a late-1940's Scott-Atwater outboard motor once, sort of like the ones used on T-Models.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!