About 3/4 of the time, just the left handed drills will do the trick for me. Snap On makes a really nice set of lh drills and extractors. And if it doesn't tap to the old thread (unlikely, but possible) just helicoil it. If the studs are that bad, use the cylinder for a guide and transfer punch the centers of the studs. Do the drilling in a drill press if you have access to one.