That plug is most likely die cast ("pot metal") Use a hammer and punch to break it up, clean out the pieces, and replace it with a rubber expansion freeze plug with a tiny vent hole drilled through the center bolt. Problem solved!
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!