M5×0.8 Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
M5×0.8 · 8.8 · dry threads
6.17 N·m
4.55 lb-ft · targets 6.2
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 | 2.39 | 1.79 | 1.77 | 2.4 kN | 5.7 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 6.17 | 4.63 | 4.55 | 6.2 kN | 11.3 kN |
| 10.9 Alloy steel, Q&T — high strength | 8.83 | 6.62 | 6.51 | 8.8 kN | 14.7 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 10.3 | 7.74 | 7.61 | 10.3 kN | 17.3 kN |
| A2-70 304 stainless, cold worked | 4.79 | 3.59 | 3.53 | 4.8 kN | 9.9 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 M5×0.8 8.8 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 6.17 N·m |
| Zinc plated, dry | 0.22 | 6.79 N·m |
| Black oxide, lightly oiled | 0.18 | 5.55 N·m |
| Machine oil on threads | 0.15 | 4.63 N·m |
| Molybdenum disulfide / anti-seize | 0.12 | 3.7 N·m |
| Waxed or PTFE coated | 0.10 | 3.08 N·m |
Torquing a lubricated bolt to the dry spec overloads it — the
3.08
N·m row and the
6.17
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 (14.18 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.