5/8-18 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 | 110 | 82.5 | 149 | 10,558 lb | 18,940 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 170 | 127 | 230 | 16,317 lb | 30,714 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 240 | 180 | 325 | 23,036 lb | 38,393 lb |
| 18-8 SS 304/18-8 stainless | 80 | 60 | 108 | 7,678 lb | 19,196 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 5/8-18 UNF Grade 5 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 170 lb-ft |
| Zinc plated, dry | 0.22 | 187 lb-ft |
| Black oxide, lightly oiled | 0.18 | 153 lb-ft |
| Machine oil on threads | 0.15 | 127 lb-ft |
| Molybdenum disulfide / anti-seize | 0.12 | 102 lb-ft |
| Waxed or PTFE coated | 0.10 | 85 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 (0.25596 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.