I have seen a HUGE chunk of aluminum pop off a transmission case because the gasket was gooped up with silicone, some got in the hole and when the bolt was run in there was nowhere for the silicone to go and....BINK.

Its a good idea to clean out any blind holes on your cases with a small drill bit turned by hand , the dirt, gasket cement & even swarf will be removed by the action of the drill. The other way is to use a nozzle on a tin of contact cleaner or WD40 but this is messy and sprays the dirt you dont want to get into your engine over everything.

Trying to drill out a broken stud or snapped bolt can also be done by hand using an Archimedes drill and a small bit to make a pilot hole. There is less chance of going off centre by this method but it is extremely time consuming.

Almost everything I tighten gets Never Seez grey antiseize paste. Better, more uniform torques, not rust or gall in place. Have some amazing bolts in highly corrosive,hot grinding environments where, outside of the never seez coated area, 3/4 of the bolt body is corroded away, yet still able to remove the nuts with a hand wrench.

Oil or water in the hole, especially verticle head bolt holes in engine block, does what charlie describes above. Don't ask.

OTherwise for me its the left hand drill bits usually, and the spline type shouldered extractors. I don't have TIG, but have done the weld trick occasionally welding a flat washer onto the stub end, then a nut to that. HRS post about applying some heat is definitely a a help.

kcj