thread repair

The stuff that I have--made by Permatex--says that it will repair and hold in spark plug threads, so it must supposedly be suitable for that kind of heat. But I have not had much success with it at all. Best use helicoils and the job is done right. OTOH, in places where you can step up a bolt size-say from 7 mm to 8 mm, you could try to redrill to the proper size for an 8 mm tap and tap the hole to fit the 8 mm bolt.