Nicely done- I'm staring down the barrel of this repair now.
I've already started to drill out the one stubborn glow plug that broke in the head.

Mine actually came out part of the way before it broke - the top 3-4 threads came out, the plug sheared across the threads when the "barrel" stuck hard in the carbon.

What about using a LH thread tap and LH thread bolt to - "bottom" into the existing portion of glow plug, then turn out the remaining portion of the GP?

So far ez outs have not worked well and I've been soaking this thing with PB blaster for quite some time now.

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95 E300 Diesel 265k and counting