Well, I did it. I broke a bolt off in the water pump. The top radiator hose comes into a housing bolted to the pump. I removed two bolts successfully, but broke the third off. I drilled into the bolt and tried a broken bolt extractor but all it did was turn in the drilled hole and it didn't bite in. So, I have a problem.
This episode has brought up a couple of points: what size hole and extractor would you use for an 8mm bolt? Largest hole possible, smallest? Now, I have a hole within the broken bolt. It seems to me that I could pass a large drill bit down the hole in the housing and drill the hole completely. Then what? The local parts store has thread repair kits for spark plugs and tap and die sets. Which works for my case? How about liquid metal compounds? As you can imagine, a small gasket repair has now ruined my day!
Thanks for any tips you can provide. I'm sure that other home mechanics would appreciate the knowledge as well.

You may want to try some heat (a little) and a left hand drill. Sometimes the leftys will grab hold and pull out the frozen bolt (especially with heat).

Landi European car repair in Parker, Colorado got the broken stud out with a lot of heat, drilling and pounding. They finally drilled the hole as much as they could to allow a thread insert to fit. Actually, replacing the water pump might have been a better idea than removing the stud. This may be another area where it would be a good idea to coat the bolts with the copper based anti-sieze compound.