Sorry didnt see til after I postedinstead of helicoils for the plug go with an insert kit from auto zone.the kit comes with a tool to smack to spread the insert.the outer diameter has spikes that bite into the surrounding metal.
What keeps the bushings in place when tensioning the plug ?I don't know what size off hand but I've built brass bushings and fixed plug holes.
Works the same as a helicoil, I put it on the plug then screw it in with loctite and run it. Not a great fix but I have used it when one is stripped badWhat keeps the bushings in place when tensioning the plug ?