Lots of good theories here- - - -now for some practical experience. Get the plug RED HOT with an acetylene torch, touch a wax candle or a block of parrafin to it (it will smoke, or possibly flame up) grab it with a pair of vise grips, and turn it out. I've removed a couple of hundred oil gallery plugs that way, and never had any problems with block damage at all. The plug has a 1/4" NPT thread, and after about half a turn the tapered thread gets very loose.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!