broken spark plug

Hey guys,
How would you go about getting a broken spark plug out of the head? When I say 'broken" I mean the top of the plug is completely gone and only the steel threads are left. You can see the cylinder top through the plug threads. I have tried using BP blaster and WD40, letting it soak for hours. That did not work. I have a BIG screw extractor and tried that also and it would not budge. I am out of ideas.

Thanks
Fratedog

So long as the screw extractor isn't digging beyond the steel plug threads into the aluminum head threads, it might help to heat the area with a torch immediately before using the screw extractor. Theory being that the aluminum head will expand more than the steel plug threads, thereby loosening the death-grip between them.

If it proves impossible to remove the broken plug without damaging the head threads beyond feasibility of re-threading, might then need to consider a heli-coil repair.

Good Luck!