#6-40 UNF 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 | 0.962 | 0.722 | 1.3 | 418 lb | 750 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 1.49 | 1.12 | 2.02 | 646 lb | 1,217 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 2.1 | 1.57 | 2.85 | 912 lb | 1,521 lb |
| 18-8 SS 304/18-8 stainless | 0.7 | 0.525 | 0.949 | 304 lb | 760 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 #6-40 UNF Grade 5 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 1.49 lb-ft |
| Zinc plated, dry | 0.22 | 1.64 lb-ft |
| Black oxide, lightly oiled | 0.18 | 1.34 lb-ft |
| Machine oil on threads | 0.15 | 1.12 lb-ft |
| Molybdenum disulfide / anti-seize | 0.12 | 0.892 lb-ft |
| Waxed or PTFE coated | 0.10 | 0.744 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 (0.01014 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.