My Chevy flatbed broke a starter mounting bolt, and after a half day of frustration trying to get the broken stub out I drilled it for a helicoil, and just to keep me from having too much fun I managed to break off the tap.  We tried welding a nut to it, vice grips, and bad words, but no good.  Yesterday I went to the tap and die shop and got one of these, and about ten minutes later it was out.  I'm sure some of you have used them before, but I hadn't, and they really are a neat tool to have when you need one.