Hi craig

I dont know how the glow plug is broken but I have had 2 instances where the glow plug body has screwed out leaving the heated element still in the hole. On ours the tip of the glow plug had been carboned up really bad. They were that stuck i thought something must have hit the end and mis-shaped it.

We tried running the engine with lots of protection around the glow plug incase it shot out and killed us when the engine warmed up and expansion took place. This didn't work.

In the end with the engine warm we modified some decent vise grips to fit in the slide hammer and clamped the element and proceeded to pull it out.

Dont twist it it will break.

I dont know if this is your problem but if it is you can still drill after or worst case scienario take the head off.

HTH

James