If it´s a cover bolt, I wouldn´t sweat it too much. Like suggested clean out the hole with a tap. If it´s questionable, you can try what I did. I rethreaded, cleaned it up good and put a stud in with a good epoxy glue. I have two studs in there, makes lining up the bolt holes with the cover smeared up with RTV an snap. Don´t have to slide the cover around looking for the holes.
Put a coat of RTV on the cover (after cleaning up the sealing surfaces real good with a solvent). Tighten the bolts finger tight (or just snug) wait a few hours or even overnight, torque the cover bolts lightly. All the bolts can fall out and the cover still won´t leak.
Loc Tite, Permatex and others make a high temp. Epoxy metal mender, that works well. Probably won´t hold without some threads to help, but will increase the chances of questionable threads holding. I only use this methode in places with little stress on the bolts, mostly for covers and cases. You can actually, lightly coat the bolt with car wax and get it back out of there, if necessary.
Blind holes are hard to tap, the tap is cone shaped, difficult to get it in the hole deep enough on occasion. Possible to have the same problem with a heli coil, tap won´t go in deep enough. There are special taps, with less of a cone shape on the end, something to keep in mind.