When im dealing with broken hardend bolts/taps i grind the surface flat take a drill index pin or a pointed chisel and punch a good center mark with a decent indention then depending on sise drill a small hole all the way thru and if your bits arnt sharp enough drilling slow with cutting oil i use a good masonary bit to drill hardend bolts when u get full depth drive your spiral easeout in the hole then when your ready to back it out heat the outside flange with a propane torch just a little then it should come on out