If it is stuck in an iron part the comments above apply. However if it is stuck in an aluminum part like a head or intake manifold an old trick is to put concentrated nitric acid on it. The aluminum will form a passive oxide coating and will not dissolve further but any iron will dissolve. I am not sure where you can get HNO3 except in a Chemistry lab and it is really dangerous stuff capable of extreme burns on skin tissue but I have seen broken drill bits dissolved out of aluminum parts using HNO3.

Don Shillady
Retired Scientist/teen rodder