Heat- - - -KROIL- - - -tap- - - -repeat as necessary.

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You could also drill and tap the plug and pull it out of the block with a bolt and nut or a slide hammer, polish it a little, and re-insert it. I'd only attempt that if I had a bare block on a stand where there was plenty of access. When I build my Cadillac 261, I'm going to pull the plug out, thread the hole, and go back in with a screw-in blockoff instead of a press fit.
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!