M18×1.5 Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
M18×1.5 · 8.8 · dry threads
339 N·m
250 lb-ft · targets 94.1
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 | 131 | 98.5 | 96.9 | 36.5 kN | 86.5 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 339 | 254 | 250 | 94.1 kN | 173 kN |
| 10.9 Alloy steel, Q&T — high strength | 485 | 363 | 357 | 134.6 kN | 224.9 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 566 | 425 | 418 | 157.3 kN | 263.8 kN |
| A2-70 304 stainless, cold worked | 263 | 197 | 194 | 73 kN | 151.4 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 M18×1.5 8.8 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 339 N·m |
| Zinc plated, dry | 0.22 | 372 N·m |
| Black oxide, lightly oiled | 0.18 | 305 N·m |
| Machine oil on threads | 0.15 | 254 N·m |
| Molybdenum disulfide / anti-seize | 0.12 | 203 N·m |
| Waxed or PTFE coated | 0.10 | 169 N·m |
Torquing a lubricated bolt to the dry spec overloads it — the
169
N·m row and the
339
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 (216.23 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.