When I first converted a ST 3000 to gas, I was running the 1/4 X 28 spark plug. It really gave a lot of problems running the engine inverted because the plug fouled so easily. I tapped the head for a 10mm plug. It turns out that the head was too thin and soft and the 10mm plug blew out the first time I ran it. I made an adapter out of brass much like the one that is being described. There was a flange inside the combustion chamber, I turned out the head so the flange would fit. The adapter was made to accept a 10mm plug. However I bored the head out so I could use a 5/8" left hand tap. The threads on the outside of the adapter were cut left handed, but the inside for the 10mm plug were standard right hand. I didn't use anything to seal or lock the threads. Installing the plug lockes the adapter into the head.