#8-36 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 | 1.66 | 1.25 | 2.25 | 607 lb | 1,089 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 2.57 | 1.92 | 3.48 | 938 lb | 1,767 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 3.62 | 2.72 | 4.91 | 1,325 lb | 2,209 lb |
| 18-8 SS 304/18-8 stainless | 1.21 | 0.906 | 1.64 | 441 lb | 1,104 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 #8-36 UNF Grade 5 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 2.57 lb-ft |
| Zinc plated, dry | 0.22 | 2.82 lb-ft |
| Black oxide, lightly oiled | 0.18 | 2.31 lb-ft |
| Machine oil on threads | 0.15 | 1.92 lb-ft |
| Molybdenum disulfide / anti-seize | 0.12 | 1.54 lb-ft |
| Waxed or PTFE coated | 0.10 | 1.28 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 (0.01473 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.