5/16-18 bolt tightening torque
A 5/16-18 bolt in SAE Grade 5, dry, torques to about 17.4 lb-ft (23.6 N·m) at 75% of proof load.
0.3125″ major dia · 18 TPI · At = 0.0524 in² · clean, undamaged threads
5/16-18 torque by grade and lubrication (lb-ft)
| Grade | Dry / as-received steel K=0.2 | Zinc plated K=0.22 | Lubricated (light oil) K=0.15 | Molybdenum disulfide K=0.1 | Anti-seize compound K=0.13 | Waxed / PTFE K=0.1 |
|---|---|---|---|---|---|---|
| SAE Grade 2 | 11.3 | 12.4 | 8.45 | 5.63 | 7.32 | 5.63 |
| SAE Grade 5 | 17.4 | 19.1 | 13.1 | 8.7 | 11.3 | 8.7 |
| SAE Grade 8 | 24.6 | 27 | 18.4 | 12.3 | 16 | 12.3 |
| 18-8 Stainless | 8.19 | 9.01 | 6.14 | 4.1 | 5.32 | 4.1 |
Proof strengths per SAE J429; tensile stress area per ASME B1.1. Values at 75 % of proof load.
Caution — match the column to your assembly. The same bolt can need half the torque lubricated versus dry, because K drops from ~0.20 to ~0.10. A dry-torque figure applied to a lubricated bolt over-tightens it and can snap it.
How the value is worked out
Torque isn't the goal — clamp load is. Tightening stretches the bolt into a controlled tension that squeezes the joint together. For a 5/16-18 in SAE Grade 8 at 75 % of proof load that target tension is about 4720 lbf; torque is just the indirect lever used to reach it, and the relationship runs through the nut factor K. The tensile stress area of the 5/16-18 thread is 0.0524 in²; multiply it by the grade's proof strength and by 0.75 to get the preload F, then feed F into T = K·D·F for the table above.
Choosing the right friction condition
This is the input people get wrong most often. K bundles every friction loss in the joint — under the head and across the threads — and it moves the torque dramatically: dry, a 5/16-18 in SAE Grade 8 wants about 24.6 lb-ft; with light oil only 18.4 lb-ft; with MoS₂ roughly 12.3 lb-ft — for the same target tension. Always match the column to what is actually on the threads.
Grade matters as much as size
SAE Grade 2 tolerates roughly 11.3 lb-ft dry; SAE Grade 8 in the same diameter takes about 24.6 lb-ft because it safely holds far more tension. Torquing a lower grade to a higher grade's table is another classic way to fail a joint — check the head markings before picking a row. And remember: torque-wrench control scatters ±25 % in practice; for safety-critical joints use angle-control or direct tension measurement.
Tapping the mating hole? 5/16-18 tap drill size · 5/16 clearance hole
References
- SAE J429 — Mechanical and material requirements for externally threaded fasteners (proof strengths).
- ASME B1.1 — Unified inch screw threads: tensile stress area.
- Machinery’s Handbook — torque–preload relation and nut-factor scatter.
Last reviewed: 2026-07 · Calculation method & assumptions