M39×4 Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
M39×4 · 8.8 · dry threads
3311 N·m
2442 lb-ft · targets 424.5
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 | 1284 | 963 | 947 | 164.7 kN | 390.3 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 3311 | 2483 | 2442 | 424.5 kN | 780.6 kN |
| 10.9 Alloy steel, Q&T — high strength | 4738 | 3553 | 3494 | 607.4 kN | 1014.8 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 5537 | 4153 | 4084 | 709.9 kN | 1190.4 kN |
| A2-70 304 stainless, cold worked | 2569 | 1927 | 1895 | 329.3 kN | 683 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 M39×4 8.8 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 3311 N·m |
| Zinc plated, dry | 0.22 | 3642 N·m |
| Black oxide, lightly oiled | 0.18 | 2980 N·m |
| Machine oil on threads | 0.15 | 2483 N·m |
| Molybdenum disulfide / anti-seize | 0.12 | 1986 N·m |
| Waxed or PTFE coated | 0.10 | 1655 N·m |
Torquing a lubricated bolt to the dry spec overloads it — the
1655
N·m row and the
3311
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 (975.75 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.