Hi my name is John.
Where is it broke off at?
Is any thing sticking out of the side of the head?
Yes there is . There two way to do this one you drill a small hole 1/8inch through the center of the glow plug and use an easy out to remove it.if you get off on the hole you buy an heia-coil set for that thread.you have to measure the thread on a different glow plug and order the heia-coil set at a parts store.Make sure you go in straight.When you get the broken glow plug out.Disable the engine so it won't start and turn the engine over to blow out any loose material out the hole.
Also left hand drill bits helps.
let me know if you need more.
please push the accept button when were done.
Thank you for choosing just answer.
snap-on sell a good remove set with a pilot,it about 85.00 dollars