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Bonnan -- Spiral Thread Inserts (STIs -- the best-known brand of STI is Heli-Coil, there are several others) are pretty much "the usual" to repair stripped threads in the automotive world. There are other methods that are often better. The biggest disadvantage to STIs is that they like to migrate. On through-holes, they'll sometimes they'll stick to the screw being installed and recess themselves into the hole or fall out from the other side of the workpiece. Sometimes they'll stick to a screw being removed and come out that way. There's another issue with STIs that comes into play if the screw is stainless: galling. The vast majority of STIs are wound from stainless wire, and they really like to gall to stainless screws. If you absolutely must use a stainless screw with a stainless STI, be sure to use a good anti-seize compound. A primary advantage of STIs is that they have a fairly small OD. A thread stripped out of a casting ear often doesn't have enough metal surrounding the hole to accept any sort of thread repair insert larger than an STI. There is one solid thread repair insert that is little, if any, larger than an STI. Its tradename is Time-Sert, and it's slick. A quick web search will turn up Time-Sert's website, and you can get a good look-see. Many performance-oriented automotive machine shops are set up to install Time-Serts, but beyond that they're pretty rare. There are two varieties of solid thread repair inserts that tend to be thickwalled but are otherwise excellent. The first, the "key-locking insert" (the best-known brand name is Keen-Sert), is turned into a larger-size tapped hole, and once in place two or more splines (the keys) are pressed along grooves on the OD of the insert through the tapped threads on the workpiece. The keys keep the insert from turning, and thereby lock it in place. The second is a simple threaded sleeve that is held in the larger-size tapped hole with adhesive. Some ready-made versions are coated with an encapsulated adhesive that is released and mixed when it rubs against the inside of the tapped hole it threads into. Others, including homemade ones, are held into the tapped hole with an epoxy or Loc-Tite-type anaerobic adhesive that is used to coat the insert at the time of installation. One common brand of this type of insert is the Re-Nu-Thread, and the major advantage beyond being strong and well locked in place, is that the receiving hole is tapped using a standard-size tap rather than a special AND that the installation tooling can be as simple as a screw and nut that fits into the insert's internal thread. Fans of the old air-cooled Volkswagens often call this last type of insert a "case saver". My suggestion is to use a case saver in your Bronco if there's room for one, otherwise try for a Time-Sert, and only resort to a STI if you're desperate. John
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