These nuts have a strip of metal spot welded to them to act as a wrench. That's what breaks loose and lets the nut spin. When you do manage to get the bolt part way out but then it gets tight it's because rust on the threads is getting inside the nut. Any professional would stop instantly, apply a spray lubricating grease, then run the bolt back in to bring the grease into the threads in the nut. Continuing to try to unscrew a bolt that gets tighter and tighter is just asking for disaster.
The trick is to reach inside the frame opening with a wire feed welder to tack the metal strap back onto the nut. As soon as that is done, you turn the bolt before the nut has had a chance to cool. As the nut cools and shrinks, the bolt will get harder to turn. At that point, you can reheat the nut with a small flame from an acetylene torch to expand it. Don't waste your time trying to do this with a propane torch. The flame is way too big, the temperature is too low, and you'll just make the nut mad!
Tuesday, April 7th, 2009 AT 10:14 AM