M39×3 Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
M39×3 · 8.8 · dry threads
3489 N·m
2574 lb-ft · targets 447.3
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 | 1354 | 1015 | 998 | 173.5 kN | 411.4 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 3489 | 2617 | 2574 | 447.3 kN | 822.7 kN |
| 10.9 Alloy steel, Q&T — high strength | 4993 | 3745 | 3683 | 640.2 kN | 1069.5 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 5836 | 4377 | 4304 | 748.2 kN | 1254.6 kN |
| A2-70 304 stainless, cold worked | 2707 | 2030 | 1997 | 347.1 kN | 719.9 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 M39×3 8.8 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 3489 N·m |
| Zinc plated, dry | 0.22 | 3838 N·m |
| Black oxide, lightly oiled | 0.18 | 3140 N·m |
| Machine oil on threads | 0.15 | 2617 N·m |
| Molybdenum disulfide / anti-seize | 0.12 | 2094 N·m |
| Waxed or PTFE coated | 0.10 | 1745 N·m |
Torquing a lubricated bolt to the dry spec overloads it — the
1745
N·m row and the
3489
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 (1028.39 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.