M2×0.4 Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
M2×0.4 · 8.8 · dry threads
0.361 N·m
0.266 lb-ft · targets 0.9
kN clamp load (75% of proof)
By grade
| Grade | Dry (N·m) | Lubed (N·m) | Dry (lb-ft) | Clamp load | Ultimate tensile |
|---|---|---|---|---|---|
| 4.6 Low-carbon steel, general purpose | 0.14 | 0.105 | 0.103 | 0.3 kN | 0.8 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 0.361 | 0.271 | 0.266 | 0.9 kN | 1.7 kN |
| 10.9 Alloy steel, Q&T — high strength | 0.516 | 0.387 | 0.381 | 1.3 kN | 2.2 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 0.603 | 0.453 | 0.445 | 1.5 kN | 2.5 kN |
| A2-70 304 stainless, cold worked | 0.28 | 0.21 | 0.206 | 0.7 kN | 1.5 kN |
Why lubrication changes everything
Roughly 90% of the torque you apply is consumed by friction — under the head and in the threads. Only about 10% becomes useful bolt stretch. That means the friction coefficient, not the bolt, sets the relationship between torque and clamp load. Here is the same M2×0.4 8.8 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 0.361 N·m |
| Zinc plated, dry | 0.22 | 0.397 N·m |
| Black oxide, lightly oiled | 0.18 | 0.325 N·m |
| Machine oil on threads | 0.15 | 0.271 N·m |
| Molybdenum disulfide / anti-seize | 0.12 | 0.216 N·m |
| Waxed or PTFE coated | 0.10 | 0.18 N·m |
Torquing a lubricated bolt to the dry spec overloads it — the
0.18
N·m row and the
0.361
N·m row produce the same clamp load. Apply the dry
number to a waxed fastener and you are at roughly twice the intended preload, which
for a 8.8 is past yield.
Assumptions behind these numbers
T = K × D × F with K = 0.20, F = 0.75 × proof load, and proof load = tensile stress area (2.07 mm²) × the grade's proof strength. This is a general-engineering starting point for a reusable steel-on-steel joint. It is not a substitute for a manufacturer's torque sequence on an engine, a suspension component, a pressure vessel or any structural connection.