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Posted by David Sherman on Tuesday, June 10, 2008 at 00:41:50 :
In Reply to: Re: Differential with broken bolts, stripped threads posted by Tim Holloway on Monday, June 09, 2008 at 23:22:23 :
I'm assuming these are really studs, not bolts, and the stripped threads are on one of the studs, not in the hole in the housing. That being the case, what you have is the usual old-truck issue of getting stuck, broken, or bunged-up bolts or studs out of whatever they're screwed into. Somebody could write a book on that subject, but the consensus is usually along these likes. Try soaking for a day in a good penetrating oil like Kroil or PB Blaster (WD 40 is NOT penetrating oil). Do not try to drill them and use an easy-out. Studs and grade-8 bolts are very hard. They'll eat any but the highest-quality drill bits, and unless you start exactly in the center, the bit will break through the side and then start chewing into the housing. If by some miracle you get a hole centered in the broken stud, you'll break the easy-out before the stud comes loose. Once in a while, you get lucky and penetrating oil works. Most of the time you need head. Often welding a nut to the top of the stud gives you enough heat to break it loose, and it gives you something big to put a wrench on. Sometimes it's necessary to get the stud red-hot at the point where it goes into the housing. When it cools off, it should break the rust loose. As it cools, but before it gets quite cool enough to touch, put a bit of wax against it. If the wax burns, wait a while. The molten wax will seep into the threads and help loosen it. Once in a while I've had a situation where that didn't work, and I had to resort to the "final solution" which is to first weld something onto the stud so I could get a hold of it, then let it get good and cold, and heat the casting around it red-hot as quickly as possible. Ideally the casting will be bright red while the bolt/stud is still dark. In this situation, the differential expansion is sure to make the bolt come loose. This is a last-ditch tactic, however, because it's sure to warp the casting or housing, and is pretty likely to crack it if it's cast. If you do have to drill a hardened stud or bolt out, heating it red hot and letting it cool slowly will reduce the hardness somewhat and make it easier to drill. There are commercial operations that use EDM (electric discharge machining) to work miracles on things like this. They can cleanly remove a hardened stud from an aluminum housing, for example. I don't know what they charge, but I suspect it's about 10% less than a new housing.
In summary, drilling is bad, heat is good.