#4-40 UNC Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
By grade
| Grade | Dry (lb-ft) | Lubed (lb-ft) | Dry (N·m) | Clamp load | Ultimate tensile |
|---|---|---|---|---|---|
| Grade 2 Low/medium carbon steel, unmarked head | 0.465 | 0.348 | 0.63 | 248 lb | 446 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 0.718 | 0.538 | 0.973 | 384 lb | 723 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 1.01 | 0.76 | 1.37 | 542 lb | 904 lb |
| 18-8 SS 304/18-8 stainless | 0.338 | 0.253 | 0.458 | 180 lb | 452 lb |
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 #4-40 UNC Grade 5 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 0.718 lb-ft |
| Zinc plated, dry | 0.22 | 0.79 lb-ft |
| Black oxide, lightly oiled | 0.18 | 0.646 lb-ft |
| Machine oil on threads | 0.15 | 0.538 lb-ft |
| Molybdenum disulfide / anti-seize | 0.12 | 0.431 lb-ft |
| Waxed or PTFE coated | 0.10 | 0.359 lb-ft |
Assumptions behind these numbers
T = K × D × F with K = 0.20, F = 0.75 × proof load, and proof load = tensile stress area (0.00603 in²) × 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.