Aircraft crankshaft bolt tensioning

The propeller bolts are about as sophisticated a bolting operation as I've ever seen, using Morgrip bolts and measuring stretch. They also measure stretch on the connecting rods on aircraft piston engines. If you've got a Lycoming engine or a Teledyne Continental engine, when they tighten up the connecting rod, the connecting rod goes with a bearing against the crankshaft, and they have two bolts holding it on. Those bolts have ground top, head, and bottom end, and they're only about two and a half inches long. You take a micrometer and measure the bolt before — it's ground to within 2.623 inches plus or minus one thousandth of an inch. After you stretch it, torque it, you don't measure the torque — well, you can approximately measure the torque, but you don't rely on torque because of all the variations in friction. You actually measure the stretch, because Young's modulus doesn't change by very much, it's a lot more precise than relying on friction.

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.