7/8" bolt torque
Tighten a 7/8-9 UNC grade 5 bolt to 429 ft-lb (582 N·m) with dry threads, or 322 ft-lb when lubricated. Grade 8 takes 606 ft-lb and grade 2 takes 167 ft-lb.
7/8" torque by thread and grade
Tap a row to show it on the dial. The unit switch above changes every value on this page.
| Thread and grade | Dry torque | Lubricated | Clamp load | Proof load |
|---|---|---|---|---|
| 7/8-9 UNC Grade 2 | 125 ft-lb | 11,428 lbf | 15,237 lbf | |
| 7/8-9 UNC Grade 5 | 322 ft-lb | 29,436 lbf | 39,247 lbf | |
| 7/8-9 UNC Grade 8 | 455 ft-lb | 41,556 lbf | 55,408 lbf | |
| 7/8-14 UNF Grade 2 | 138 ft-lb | 12,609 lbf | 16,813 lbf | |
| 7/8-14 UNF Grade 5 | 355 ft-lb | 32,479 lbf | 43,305 lbf | |
| 7/8-14 UNF Grade 8 | 502 ft-lb | 45,853 lbf | 61,137 lbf |
Clamp load is 75 % of proof load. Dry: K = 0.20. Lubricated: K = 0.15.
7/8" at a glance
Wrench, drill and thread data for the same bolt.
Before you tighten
Lubricant changes the number
Oil, moly paste or anti-seize lowers the nut factor K, so the same torque produces far more clamp load. Use the lubricated value whenever the threads are oiled, or the bolt can yield.
Tighten in a star pattern
On flanges and covers, bring every bolt to about 30 %, then 70 %, then full torque, crossing the pattern each pass. Finish with one round at full torque in sequence.
Give the thread enough grip
Engage at least one diameter of thread in steel, about 1.5 diameters in cast iron and 2 diameters in aluminium, or the internal thread strips before the bolt reaches its clamp load.
How the number is worked out
The 7/8-9 grade 5 figure, step by step.
- K
- 0.20, nut factor for dry plain steel
- F
- 29,436 lbf, 75 % of the 39,247 lbf proof load
- d
- 0.8750 in, nominal diameter
Where the inputs come from
Proof strength is taken from SAE J429: 33,000 psi for grade 2 at this size, 85,000 psi for grade 5 and 120,000 psi for grade 8. Multiply by the tensile stress area to get the proof load.
The stress area comes from ASME B1.1: As = 0.7854 × (D − 0.9743 / n)². Target clamp load is 75 % of proof load.
Lubricated values use K = 0.15. For other coatings or preload targets, use the torque calculator.
Other inch sizes
Common questions
What is the torque for a 7/8 grade 5 bolt?
About 429 ft-lb dry for 7/8-9 UNC and 474 ft-lb for 7/8-14 UNF, based on 75 % of proof load.
What is the torque for a 7/8 grade 8 bolt?
About 606 ft-lb dry for 7/8-9 UNC and 669 ft-lb for 7/8-14 UNF.
What is the torque for a 7/8 grade 2 bolt?
About 167 ft-lb dry for 7/8-9 UNC.
What size wrench fits a 7/8 bolt?
A standard 7/8" hex cap screw takes a 1-5/16" wrench or socket.