reducing hole size too big for helicoil

[Mike E.]

If you have a lathe (?) make the plug with a shoulder on it so that it will jamb once seated rather than possibly travel all the way through when installing. if the shoulder is made wide, a small hole can be drilled through the face and a set screw or roll pin used to prevent it unscrewing if you have to remove the 5/16" bolt. I used this method for fixing a stripped out helicoil on a spark plug hole in a motorcycle engine head.