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Tom S. is right on the money. Also if you notice, most are fine thread, when you apply 70 ft/lbs of turn torque, you are putting up to 20 to 30,000 lbs of tension on a grade 5, 1/2" x 20. This deforms the threads, although not usually noticed by the eye, by backing off and reusing some bolts, you are actually slightly cross threading and the torque will no hold. Some bolts are different, when you get to plastic and elastic (not the material) things change. One example of reusable bolts that are torqued and reused are head bolts in our engines, although it is recommended, just like our u bolts to recheck torque after the first 500 to 1000 miles of use. Many of the newer engines have a head bolt that is called TTY(torque to yield) they stretch and then become solid, these should not be reused. A lot of information just to insure we use the proper fastener, but really important. Hence the interest in this thread.
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