30-10-11, 10:36 AM
With the head off you could knock any remaining ceramic out so it's hollow and weld up the hole, as the weld cools it will shrink the thread by a few thou making it looser, weld a nut to the top and away you go, bit of a git having to take the head off but as said before that ceramic is hard as feck and going to do lots of damage to the engine if left in.
Save the planet...It's the only one with beer!