John, when you tighten a bolt there are three things to consider. Firstly some of the torque must overcome thread friction. Then some more is taken up by friction between the nut and the adjacent surface. The remainder stretches the bolt as it runs up the thread. In a typical, un-oiled fastener they are roughly thirds, but to stretch the bolt and provide the clamping force you must first overcome thread friction and under head friction. So, with less thread friction, oiled perhaps , the bolt stress is increased. With a stiff nut it’s reduced. This is important. It’s very easy to put an oiled fastener into yield if the quoted torque figure is for a dry joint. Likewise a rusty nut will be stiff and will absorb a lot more torque, reducing the clamping load. If you deduce from this that torque tightening isn’t reliable then you would be right. Partly. It’s better than nothing, though. Some tests we once did showed that experienced operators working by feel only were a lot less consistent than untrained ones with a torque wrench. But, I agree with Zeto. What you really want to be doing is measuring stretch.
Alan Fairless