[D. Jones]

My usual way to remove broken bolts from cast iron is to TIG weld a nut to the remains of the bolt, getting the bolt red hot, then let it cool and slowly wiggle it back and forth using either wax or one of the better penetrating oils. The heat helps break the bolt loose. Sometimes the nuts twist off and it takes welding several nuts on to get the remains of the bolt out. Most people don't have a TIG welder, but any good welding shop will.