Understanding Bolt Torque
When you tighten a bolt, the number on your torque wrench is not really the point. Torque is only a means to an end, and that end is preload — the tension stretched into the bolt shank that clamps two parts together. A properly tensioned joint carries its working load through friction between the clamped faces, not through the bolt itself, which is why getting the torque right matters far more than most people realise.
What torque actually does
Turning the nut stretches the bolt like a very stiff spring. That stretch stores tension, and the tension pulls the joint faces together with a clamping force. As long as the external load on the joint stays below that clamping force, the parts never separate and the bolt sees almost no fluctuating stress. Under-tighten and the joint can work loose or fail by fatigue; over-tighten and you yield or snap the bolt. The whole craft of a bolted joint is landing the preload inside the right window.
The torque-tension equation
The relationship engineers use every day is short:
T = K × F × d
Here T is the applied torque, F is the preload (clamp force) you want in the bolt, d is the nominal bolt diameter, and K is the nut factor, a dimensionless number that lumps together all the friction in the threads and under the turning face. Notice what this says: torque and tension are not the same thing. The wrench measures torque, but what actually clamps the joint is the tension F, and K is the messy bridge between the two.
Why K matters so much
Most of the effort you put into a wrench never becomes clamp force at all. In a typical dry steel joint, only around 10 to 15 percent of your torque turns into bolt tension; the rest is burned fighting friction under the head and in the threads. Because K sits directly in the equation, it dominates the result:
- K ≈ 0.20 for a plain, dry, as-received steel fastener.
- K ≈ 0.15 when the threads are lubricated with oil or anti-seize.
- K can fall to 0.10 or lower with specialised coatings such as wax or moly.
The practical warning follows immediately: lubricating a bolt but using the dry torque figure produces far more preload than intended — often enough to yield the bolt. Always match the torque spec to the actual condition of the threads.
Setting the target preload
Preload is not chosen at random. It is pegged to the bolt's proof load, the highest tension the bolt can carry without taking a permanent set. A common rule for reusable joints is to preload to about 75 percent of proof load, which clamps hard while leaving a margin below yield. The proof load itself depends on the bolt's property class and its stress area, so a higher-grade bolt of the same diameter can safely be tightened to a higher preload.
Typical tightening torques
The table gives rough dry-thread tightening torques for coarse-thread steel bolts at roughly 75 percent proof load. Treat them as ballpark figures — always defer to the manufacturer's specification.
| Size | Class 8.8 (N·m) | Class 10.9 (N·m) |
|---|---|---|
| M6 | 10 | 14 |
| M8 | 25 | 35 |
| M10 | 49 | 69 |
| M12 | 85 | 120 |
| M16 | 210 | 295 |
Worked example
Suppose you want to clamp a joint with an M10 bolt (d = 10 mm = 0.010 m) to a preload of 25,000 N, using dry threads so K = 0.2. The required torque is:
T = 0.2 × 25,000 × 0.010 = 50 N·m.
Now oil the same threads so K drops to 0.15. If you still dial in 50 N·m, the preload rises to F = T / (K × d) = 50 / (0.15 × 0.010) ≈ 33,300 N — a third more tension than planned, and quite possibly past the bolt's yield point. Same torque, very different clamp force. That single example captures why torque and tension must never be treated as interchangeable.
When torque is not accurate enough
Because K can vary by 30 percent or more from bolt to bolt, torque control alone typically holds preload to only about ±25 to 30 percent. Critical joints — cylinder heads, connecting rods, structural steelwork — use tighter methods instead: turn-of-the-nut (angle) control, tightening past yield, or measuring bolt stretch directly with a gauge. These bypass the friction guesswork by watching the bolt's actual elongation rather than the torque it took to get there.
Torque figures are quick to work out once you know the nut factor and the target preload. Run your own numbers with the Bolt Torque Calculator on MechKit.