Table
Tightening torques
Guide values for metric coarse thread M4 to M20 by property class 8.8, 10.9 and 12.9, with a converter and notes on aluminium and extensions.
These are guide values for dry steel bolts in steel threads. Aluminium housings, magnesium, lubricated or thread-locked bolts, stretch bolts, spark plugs, axles and anything for which your workshop manual states a value: there the manual alone applies. When in doubt, use less and go by feel.
| Thread | 8.8 Nm | 10.9 Nm | 12.9 Nm |
|---|---|---|---|
| M4 | 3.0 | 4.4 | 5.1 |
| M5 | 5.9 | 8.6 | 10.0 |
| M6 | 10.1 | 14.9 | 17.4 |
| M8 | 24.6 | 36.1 | 42.2 |
| M10 | 48.0 | 71.0 | 83.0 |
| M12 | 84.0 | 123.0 | 144.0 |
| M14 | 133.0 | 195.0 | 229.0 |
| M16 | 206.0 | 302.0 | 354.0 |
| M18 | 295.0 | 421.0 | 492.0 |
| M20 | 415.0 | 592.0 | 692.0 |
How to identify the property class
It is stamped on the bolt head as a number: 8.8, 10.9, 12.9. The first number times 100 is the tensile strength in N/mm², the product of both numbers times 10 is the yield strength. If there is no marking, it is usually a soft bolt of class 4.6 or 5.6, and the values in the table are far too high.
Extension on the torque wrench
A spark plug spanner or a hook spanner sitting on the torque wrench as an extension lengthens the lever. The wrench then releases too early and the part ends up too tight. The torque converter works out the setting: target torque times wrench length divided by wrench length plus extension.
Aluminium
Engine housings, cylinder heads and covers on mopeds are made of aluminium. There the thread is the weakest link, not the bolt. Rule of thumb from practice: stay well below the steel value, prefer bolts with a long thread engagement, and repair a thread once it is stripped with a thread insert rather than tightening it further.
Questions
Does the table apply to spark plugs?
No. Plugs are tightened to the plug manufacturer's specification: finger tight plus a defined part turn, depending on the sealing washer and thread size. See changing a spark plug.
What does oil on the thread change?
Oil lowers the friction. At the same torque the preload rises considerably and the bolt can be overstretched. Table values apply dry; reduce them substantially when lubricated.
Is feel enough instead of a torque wrench?
For M6 in aluminium, feel is often better than a badly calibrated wrench. For the cylinder head, flywheel nut, axles and brakes: use the wrench, and the manual value.