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.