M20×1.5 Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
M20×1.5 · 8.8 · dry threads
472 N·m
348 lb-ft · targets 118.1
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 | 183 | 137 | 135 | 45.8 kN | 108.6 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 472 | 354 | 348 | 118.1 kN | 217.2 kN |
| 10.9 Alloy steel, Q&T — high strength | 676 | 507 | 499 | 169 kN | 282.4 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 790 | 593 | 583 | 197.5 kN | 331.2 kN |
| A2-70 304 stainless, cold worked | 367 | 275 | 270 | 91.6 kN | 190.1 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 M20×1.5 8.8 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 472 N·m |
| Zinc plated, dry | 0.22 | 520 N·m |
| Black oxide, lightly oiled | 0.18 | 425 N·m |
| Machine oil on threads | 0.15 | 354 N·m |
| Molybdenum disulfide / anti-seize | 0.12 | 283 N·m |
| Waxed or PTFE coated | 0.10 | 236 N·m |
Torquing a lubricated bolt to the dry spec overloads it — the
236
N·m row and the
472
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 (271.5 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.