[PeteLeathersac]

Lots of good ideas but be careful heating the plug itself as too much expansion could cause the head casting to crack? . Usually with seized items you need to heat the outside one of the two pieces (in this case the head) to allow it to expand beyond the inside piece (plug). . Have a straw or something w/ a tiny amount of water you can quickly put into the plug for a moment then immediately attempt to loosen it. . The water in the plug will cool and shrink it slightly smaller than the head and may allow you the difference for it to let go? .

JB's above mentioned trick of trying to tighten to get it to give first is a good one and often works well as it doesn't destroy the grabbing edge of the direction you need to take something off. .

Last resort, try one of the impact-driver tools. . These work great on seatbelt bolts etc. that seize but don't matter what happens to the area but w/ your head situation you need to be careful as the instant impact can carry through and cause a crack. .

I think a combination of all of the above may do it....Good Luck! .

~ Pete