Several say here to heat the bolt. I say no. You heat the surrounding area, to expand the locking threads. Most of the time the reason a bolt breaks here is because the shoulder of the bolt is locked into corrosion from the pump or timing cover. I agree with the other options of removal but would like to add one more, if other ways do not work. Set a spacer on you drill bit to go as deep as the pump or cover, less 3/8 of an inch. That should remove the area of the corroded parts. Then you should be able to use vice grips to remove the rest of the bolt from the block. I always use a anti-seize in this application on the shoulder of the bolts.
