broken spark plug

[Steelbuddha]

This old dirtbag's trick: 1) Mix up your own penetrant from 50/50 acetone and transmission fluid. (Penetrates better than any commercial product.) Repeatedly treat the stuck fastener/sparkplug over the course of a day or so. 2) Put a blunt chisel in an air hammer and rattle the fastener/sparkplug. You may be surprised.

 

[Dejure]

Stupid old farts have been known to heat things just enough to melt candle wax, then let it wick into the threads to get bolts out. They thought just because it works it was a good idea.