I need to rethread my thermastat

If the other bolt is still ok, take it to an auto parts store and ask for the proper size Heli-coil kit. They will match it up for you. It includes the proper size drill bit to drill out the hole, the proper size tap to rethread the hole larger than it was before, and the proper size insert to screw in. The insert will have the same inside thread as the original hole was. They can provide the right size replacement bolt too.

Before screwing in the insert with the included tool, use a vacuum cleaner or a piece of wire dipped in grease to get all of the metal chips out of the hole. Chips that get stuck in the bottom can hit the bolt and make it appear to be tight when it really isn't.

Caradiodoc