Torque is the clamp force scaled by size and friction
T is the tightening torque, D the bolt diameter, and F the clamp force (preload) you want. K is the nut factor, a single number that rolls up all the friction. K is around 0.20 dry, 0.15 lightly oiled, 0.10 with anti-seize. Only F does useful work; the friction that K captures eats most of the torque, which is why the same setting gives different results on different bolts.
Typical torque, plain steel, dry
| Bolt | Grade 8.8 | Grade 10.9 | Grade 12.9 |
|---|---|---|---|
| M6 | 10 N·m | 14 N·m | 17 N·m |
| M8 | 25 N·m | 35 N·m | 41 N·m |
| M10 | 49 N·m | 69 N·m | 83 N·m |
| M12 | 86 N·m | 120 N·m | 145 N·m |
| M16 | 210 N·m | 295 N·m | 355 N·m |
| M20 | 410 N·m | 580 N·m | 690 N·m |
Values assume K = 0.20 and preload near 90% of yield. Stainless and aluminum bolts are weaker and take much less; always check the grade stamped on the head.
Why lubricant changes the answer
The same torque on a lubricated bolt reaches a higher clamp force, because less of it is lost to friction. Torque set for a dry bolt can over-stretch an oiled one.
Lower friction means less torque is needed for the same clamp — so a fixed 50 N·m on a slippery thread pushes the bolt much closer to breaking.
What throws the number off
- Friction dominates. Roughly 90% of the torque fights friction under the head and in the threads. Small changes in surface condition move the clamp force a lot.
- Preload relaxes. A joint can lose 20 to 30% of its clamp force in the first day, more with soft materials. Re-torque or use a locking method.
- Over-tightening fails too. Past yield the bolt stretches permanently and fatigues faster. Tighter is not safer.
Common questions
What torque should an M10 bolt get?
About 21 N-m for a plain steel bolt clamped to around 75% of yield, dry. A high-strength grade 10.9 bolt takes far more (roughly 50 N-m), so the grade matters as much as the size.
Does oil on the threads change the torque?
Yes, a lot. Lubricant lowers the friction factor, so the same torque produces more clamp force. Applying the dry torque figure to an oiled bolt can overload it by 20 to 40%. Use the value that matches the condition of the threads.
Why does the same torque give different clamp force each time?
Because most of the torque is spent overcoming friction, not stretching the bolt. Surface finish, coating, dirt and rust all shift the friction factor, so torque alone is only accurate to roughly plus or minus 25%.
