How tight should a bolt be? Tightening torque in Nm
When you tighten a bolt with a torque wrench, you need to know the torque that provides the desired preload. Too little — and the bolt can loosen over time. Too much — and you risk damaging the threads or snapping the bolt.
Torque (Nm) is really a practical shortcut for achieving the correct clamping force in a joint. It’s the clamping force that holds the parts together — not the torque itself. But because clamping force is difficult to measure directly, we use Nm tables as guidance.
In the tables below you can look up how tight the most common metric bolts (M2 to M20) should be tightened. The value mainly depends on: the bolt size (the M number) and its strength class (e.g., 8.8 or 10.9). The higher the strength class, the more the bolt can withstand.
What do 8.8, 10.9 and 12.9 mean? It’s the strength class (how strong the bolt/screw is). It’s typically stamped on the head (e.g., 8.8 or 10.9). Higher numbers usually mean the bolt can be tightened harder – but only if the nut/tapped hole and the surrounding materials can also handle it.
Table: Tightening torque for standard bolts (coarse thread)
The table applies to ordinary steel bolts with standard thread pitch (coarse thread). When the dimension is given as M8 (without specifying thread pitch), it usually means M8×1.25 (standard coarse thread). Fine thread is typically written as, for example, M8×1.0. The values are for untreated, lightly oiled bolts tightened with a torque wrench.
| Bolt | Thread pitch mm |
8.8 Nm |
10.9 Nm |
12.9 Nm |
|---|---|---|---|---|
| M2 | 0.40 | 0.35 | 0.49 | 0.58 |
| M2.5 | 0.45 | 0.70 | 0.98 | 1.2 |
| M3 | 0.50 | 1.2 | 1.7 | 2.1 |
| M3.5 | 0.60 | 1.9 | 2.7 | 3.3 |
| M4 | 0.70 | 2.9 | 4.0 | 4.9 |
| M5 | 0.80 | 5.7 | 8.1 | 9.7 |
| M6 | 1.00 | 9.8 | 14 | 17 |
| M7 | 1.00 | 16.5 | 23 | 27 |
| M8 | 1.25 | 24 | 33 | 40 |
| M10 | 1.50 | 47 | 65 | 79 |
| M12 | 1.75 | 81 | 114 | 136 |
| M14 | 2.00 | 128 | 181 | 217 |
| M16 | 2.00 | 197 | 277 | 333 |
| M18 | 2.50 | 275 | 386 | 463 |
| M20 | 2.50 | 385 | 541 | 649 |
Source: Nordic Fastening Group AB.
Table: Bolts with fine thread
Some bolts have a finer thread than normal — the thread “steps” are closer together. It is written, for example, as M10×1.25 instead of the normal M10×1.5. Fine thread is often used in machines and vehicles, but it is not a guarantee against something loosening due to vibration. The most important thing against loosening is typically correct clamping force and possibly locking (locknut, threadlocker, Nord-Lock, etc.).
| Bolt | Thread pitch mm |
8.8 Nm |
10.9 Nm |
12.9 Nm |
|---|---|---|---|---|
| M8 × 1 | 1.00 | 25 | 35 | 42 |
| M10 × 1.25 | 1.25 | 48 | 68 | 81 |
| M12 × 1.25 | 1.25 | 85 | 120 | 144 |
| M14 × 1.5 | 1.50 | 135 | 190 | 228 |
| M16 × 1.5 | 1.50 | 204 | 287 | 344 |
| M18 × 1.5 | 1.50 | 294 | 413 | 496 |
| M20 × 1.5 | 1.50 | 408 | 574 | 688 |
Source: Nordic Fastening Group AB – Tightening torque.
Four things that can change the numbers
1. Oil or lubricant on the threads
The tables above apply to lightly oiled bolts. If you use a heavier lubricant (e.g., copper grease or MoS2 grease), the threads slip more easily. That means the same Nm value produces more clamping force — and you may overload the bolt. Conversely, a completely dry, untreated bolt typically produces less clamping force at the same torque.
2. Surface treatment (zinc plating, coating, etc.)
Surface treatment can change friction in either direction. Some coatings “slip” more, others “bind” more. Therefore, it’s best to use a table/specification that matches the surface you actually have.
3. The screw head, washers, and the material you tighten against
Even if the bolt can handle the torque, the material under the bolt head or nut can yield — for example soft metal, plastic, or thin sheet. In those cases, a washer under the head and a lower torque are often better than tightening “all the way.”
4. Bolts with a flat or countersunk head
Bolts with a countersunk head (DIN 7991/ISO 10642) or low button head (ISO 7380) are not as strong as bolts with a normal head. For screws with countersunk or low heads, the manufacturer may specify a reduced installation torque. Follow the manufacturer’s data. If you don’t have data, start lower than the table and check the joint.
Always use a torque wrench (especially for small bolts). Small bolts like M2–M5 have very low torque values. Here it’s difficult to “feel it out” by hand. A torque wrench in the right range can save you from stripped threads and snapped bolts.
What does this have to do with magnets?
Many pot magnets have metric threads (often M3, M4, M5, M6 or M8). When you screw a bolt or hook into a pot magnet with an internal thread, it’s a good idea to:
- Tighten carefully (often below the table’s maximum).
- Avoid “bottoming out” the screw hard in the threaded hole.
- Consider a mild locking method (e.g., a suitable threadlocker) if something needs to stay put without being tightened extremely hard.
Magnets (and especially the magnetic material) can be hard but brittle, so overzealous tightening can cause damage to the magnet itself or its assembly.
Do you need to find the right nut or drill for your thread? See our thread size chart.