1-8 UNC Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
1-8 UNC · Grade 5 · dry threads
644 lb-ft
873 N·m · targets 38616
lb clamp load (75% of proof)
By grade
| Grade | Dry (lb-ft) | Lubed (lb-ft) | Dry (N·m) | Clamp load | Ultimate tensile |
|---|---|---|---|---|---|
| Grade 2 Low/medium carbon steel, unmarked head | 250 | 187 | 339 | 14,992 lb | 44,825 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 644 | 483 | 873 | 38,616 lb | 72,689 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 909 | 681 | 1232 | 54,517 lb | 90,861 lb |
| 18-8 SS 304/18-8 stainless | 303 | 227 | 411 | 18,172 lb | 45,430 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 1-8 UNC Grade 5 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 644 lb-ft |
| Zinc plated, dry | 0.22 | 708 lb-ft |
| Black oxide, lightly oiled | 0.18 | 579 lb-ft |
| Machine oil on threads | 0.15 | 483 lb-ft |
| Molybdenum disulfide / anti-seize | 0.12 | 386 lb-ft |
| Waxed or PTFE coated | 0.10 | 322 lb-ft |
Torquing a lubricated bolt to the dry spec overloads it — the
322
lb-ft row and the
644
lb-ft 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 Grade 5 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 (0.60575 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.