Message posted by habu-e6 on September 29, 2007 at 17:37:01 PST:

This is very interesting, the fact that the technician used a die to (clean up) the threads on the bolt. As Air Force machinist we would never be allowed to do that. The problem is that most if not all aircraft bolts and screws have formed threads. They are then followed up with heat treating and plating. The internal grain structure in the bolt is then secure and follows the formation of the thread pattern inside the bolt allowing it to be much stronger than if the threads were cut.
Re-cutting the threads on the bolt cut into the grain structure pattern and weaken it severely.
A similar problem happens when running a tap through a nut plate. Except what happens is the tap cuts away the formed and heat treated locking portion of the nut. It then no longer functions as it should and a bolt or screw can simply vibrate loose.
Another problem causing broken bolts or screws is over torque. It puts undue strain on the fastener and during the extra strain put on it from operation; it can pop the head right off. I’ve seen this happen on SR-71 forward by-pass door seal fasteners. The J58s don’t seem to mind it too much though; there like Mikey they’ll eat anything.

In Reply to: D-21B anniversary posted by Peter Merlin on September 28, 2007 at 9:06:38 PST:

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