I would be tempted to pull the plug out and do a 1/3 turn clean up of the thread in the head and then do the same on the plug. Make sure that the threads are clean. Now use a good Hi Temp thread lock and screw it down really tight.

Question, is the 1/8 pipe material steel or aluminum. You may have a rough time getting a seal to last with all of the thrermal cycling an engine goes through, especially if the two materials have a different thermal expansion. You may be better served getting a new head. It may be the least expensive option after trying different compounds and possibly getting some of them in on top of the piston.



Don