Tapping broken glow plugs for removal - most suitable thread for 5.6mm ID hollow plug core?

[AndyStobbs]

Give it a bash, I had 3 foot breaker on a spark plug that wouldn't shift, filled the recess with plus gas and got it warm, came out like it was hand tight

Haha just reminded me of the last time I had a difficult spark plug. 1,2 and 3 came out ok, no. 4 looked like a mushroom and scraped and scroupped its way out taking out the thread as it came. Engine did have a bad missfire prior to this. Think the lack of piston and valves on no.4 was contributory. I found a valve stem in the turbo casting, it had curved round to the shape of the casting.