use heat as said, then apply a simple birthday candle to the threads. it is amazing how good this works ! i used this method to remove the original, tiny, ruined screwdriver head slot screws for the cowl vent screen on my 33 dodge humpback panel truck. when cooled, those screws turned right out without ruined threads on either the screws, or the female vent threads.
have used this on front leaf spring bolts as well.