`Aircraft crankshaft bolt tensioning`

Has anybody ever used a torque wrench on a bolt? What do you think the accuracy of that is? Pretty bad — plus or minus 30 percent in some cases. But that's what people use when they're trying to be precise. To get good fatigue strength in the bolt, you want the bolt tensioned to 70 to 90% of its yield strength. And you've got plus or minus 30% as your ability to do it. So in really critical applications — I first saw it on the crankshaft of an airplane engine — they actually grind the tip and the bottom of the bolt to an exact dimension, and someone takes a micrometer, and the bolt is 2.62 inches before they tension it and 2.68 inches after — stretched six tenths of an inch. Young's modulus is pretty well known, and you can get down to plus or minus 10% of your stress in your bolt, which is what you really need.