[Wayne 962]

Torque values are indeed *very* crude. What you are really trying to measure is the total stretch of the bolt - not the torque. The torque is just one way of figuring out the total stretch.

If the nuts are rusted, or the surfaces are dry or lubed, it will significantly change the stretch of the bolt, even though the torque setting will be the same. Plus/minus 3% is probably more than good enough for just about anything. If you want higher accuracy, you wouldn't use a torque wrench anways, but instead a stretch measurement gauge.

I would guess that changes in temperature, lubrication, angle, variations in mechanical wrenches, etc. can probably add up to varying torque values of 20% or more!

The best wrenches are the primative long-bar ones. These never go out of calibration, (because the properties of the metal doesn't change), and they are typically very accurate over time. They are a pain to work with sometimes though...

-Wayne