If you can center drill them accurately with successively larger holes, you can pick out the pieces and save the original threads. If not, you’ll have to drill enough out to remove the screw and use a HeliCoil.
One method of increasing your chances of drilling accurately is to make a steel plate with the exact hole pattern of the block you’re working on. Drill the holes in the plate so that the holes for good threads are the size of the bolt, the holes for the bad threads are much smaller. Then bolt the plate to the block and use it as a guide to center drill the broken bolts. Once you have the small holes drilled, use larger and larger drills until you can remove the pieces of the broken bolt.
Time is your friend here. Be patient.
T
