How much tightening torque do you need?

An impact wrench tightens with repeated blows, so the tightening torque that reaches the bolt changes with friction, the type of joint and how long it impacts. Bossard puts the scatter of impact tightening at ±43% to ±60% and calls it suitable for preliminary tightening only. For a joint with a specified torque, use a torque wrench or the method the project specifies.

Oil lowers friction in the thread, so the same tightening torque gives more tension in the bolt. A lightly oiled bolt therefore needs about 25% less tightening torque than a dry one (K = 0.15 against 0.20). Tightening an oiled bolt to the dry figure can over-tighten it and break it.

VDI and Norbar tables load the bolt to 90% of its yield strength, while we calculate at 75% of the proof load, as the Fastenal table does. Their tables therefore give up to 25% higher numbers for oiled bolts, and the gap for dry bolts is smaller (within about 10%).

Tightening torque is what the tool applies when it tightens a bolt. Nut-busting torque is a separate, usually higher figure for freeing stuck nuts. This page compares only the published tightening torque, because that is the one that matters for tightening a bolt.