M4 bolt tightening torque
A M4 bolt in Class 10.9, dry, torques to about 4.37 N·m (3.22 lb-ft) at 75% of proof load.
4 mm major dia · 0.7 mm pitch · At = 8.78 mm² · clean, undamaged threads
M4 torque by grade and lubrication (N·m)
| Grade | Dry / as-received steel K=0.2 | Zinc plated K=0.22 | Lubricated (light oil) K=0.15 | Molybdenum disulfide K=0.1 | Anti-seize compound K=0.13 | Waxed / PTFE K=0.1 |
|---|---|---|---|---|---|---|
| Class 8.8 | 3.05 | 3.36 | 2.29 | 1.53 | 1.99 | 1.53 |
| Class 10.9 | 4.37 | 4.81 | 3.28 | 2.19 | 2.84 | 2.19 |
| Class 12.9 | 5.11 | 5.62 | 3.83 | 2.55 | 3.32 | 2.55 |
| A2-70 Stainless | 2.37 | 2.61 | 1.78 | 1.19 | 1.54 | 1.19 |
Proof strengths per ISO 898-1 (ISO 3506-1 for A2-70); tensile stress area per ISO 898-1. Values at 75 % of proof load.
Caution — match the column to your assembly. The same bolt can need half the torque lubricated versus dry, because K drops from ~0.20 to ~0.10. A dry-torque figure applied to a lubricated bolt over-tightens it and can snap it.
How the value is worked out
Torque isn't the goal — clamp load is. Tightening stretches the bolt into a controlled tension that squeezes the joint together. For a M4 in Class 12.9 at 75 % of proof load that target tension is about 6.39 kN; torque is just the indirect lever used to reach it, and the relationship runs through the nut factor K. The tensile stress area of the M4 thread is 8.78 mm²; multiply it by the grade's proof strength and by 0.75 to get the preload F, then feed F into T = K·D·F for the table above.
Choosing the right friction condition
This is the input people get wrong most often. K bundles every friction loss in the joint — under the head and across the threads — and it moves the torque dramatically: dry, a M4 in Class 12.9 wants about 5.11 N·m; with light oil only 3.83 N·m; with MoS₂ roughly 2.55 N·m — for the same target tension. Always match the column to what is actually on the threads.
Grade matters as much as size
Class 8.8 tolerates roughly 3.05 N·m dry; Class 12.9 in the same diameter takes about 5.11 N·m because it safely holds far more tension. Torquing a lower grade to a higher grade's table is another classic way to fail a joint — check the head markings before picking a row. And remember: torque-wrench control scatters ±25 % in practice; for safety-critical joints use angle-control or direct tension measurement.
Tapping the mating hole? M4 tap drill size · M4 clearance hole
References
- ISO 898-1 — Mechanical properties of fasteners: property classes 8.8 / 10.9 / 12.9 (ISO 3506-1 for A2-70).
- ISO 724 / ISO 898-1 — metric thread dimensions and tensile stress area.
- Machinery’s Handbook — torque–preload relation and nut-factor scatter.
Last reviewed: 2026-07 · Calculation method & assumptions