ENGINEERING TOOL
7/8" Bolt Torque Chart (SAE Grades)
Tightening torque ranges (min–typical–max) for 7/8" bolts in SAE J429 Grade 2, 5 and 8, across five friction states — UNC 9 TPI plus the fine-thread series. K-factor method, with the metric comparison on the same page.
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Other inch sizes
Nearest metric size: M22 M22 →
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UNC Torque by Grade and Friction State
UNC 9 TPI. Tensile stress area At = 298 mm², per the ASME B1.1 formula. Target preload = 75% of the SAE J429 proof stress × At.
Quick answer: 7/8"-9 Grade 5 · electro zinc ≈ 495 N·m (365 ft·lb); other grades and friction states below.
| Grade | Proof (MPa) | Preload (kN) | Dry / Black K 0.18–0.22 · N·m | Electro Zinc K 0.15–0.2 · N·m | Hot-Dip Galv. K 0.18–0.25 · N·m | Oiled K 0.12–0.15 · N·m | Wax / PTFE / Anti-seize K 0.08–0.12 · N·m |
|---|---|---|---|---|---|---|---|
| Grade 2 | 228 | 50.9 | 226 204–249 | 192 170–226 | 249 204–283 | 147 136–170 | 113 90–136 |
| Grade 5 | 586 | 131 | 582 524–641 | 495 437–582 | 641 524–728 | 379 349–437 | 291 233–349 |
| Grade 8 | 827 | 184.9 | 822 740–904 | 699 617–822 | 904 740–1028 | 534 493–617 | 411 329–493 |
Cell shows the typical value; the small line below is the range (min–max).
References
- SAE J429 — Mechanical and Material Requirements for Externally Threaded Fasteners (published grade proof stresses)
- ASME B1.1 — Unified Inch Screw Threads (tensile stress area formula)
- Machinery's Handbook — SAE J429 proof stress tiers and empirical K-factor ranges
- ISO 16047:2005 — Fasteners: torque/clamp force testing (K-factor measurement method)
- Machine Design — anti-seize compound friction test (K≈0.10)
- ISO 898-1:2013 — Mechanical properties of fasteners made of carbon and alloy steel (used for the ISO comparison)
UNF Fine-Thread Torque
7/8"-14 UNF: same nominal diameter, finer thread, so the tensile stress area is larger — At = 329 mm² against 298 mm² for the coarse thread — and the torque is higher at the same grade and friction state.
| Grade | Proof (MPa) | Preload (kN) | Dry / Black K 0.18–0.22 · N·m | Electro Zinc K 0.15–0.2 · N·m | Hot-Dip Galv. K 0.18–0.25 · N·m | Oiled K 0.12–0.15 · N·m | Wax / PTFE / Anti-seize K 0.08–0.12 · N·m |
|---|---|---|---|---|---|---|---|
| Grade 2 | 228 | 56.1 | 249 225–274 | 212 187–249 | 274 225–312 | 162 150–187 | 125 100–150 |
| Grade 5 | 586 | 144.5 | 643 578–707 | 546 482–643 | 707 578–803 | 418 386–482 | 321 257–386 |
| Grade 8 | 827 | 204.1 | 907 817–998 | 771 680–907 | 998 817–1134 | 590 544–680 | 454 363–544 |
Cell shows the typical value; the small line below is the range (min–max).
75% vs 90% of Proof Stress
Same grade, same friction state (electro zinc, K 0.15–0.20): the standard level targets 75% of the proof stress — reusable and re-torqueable — while the yield level targets 90%, which sits close to yield and is a one-time setting.
| Grade | 75% proof (reusable) N·m | 90% proof (near yield) N·m |
|---|---|---|
| Grade 2 | 192 | 231 |
| Grade 5 | 495 | 594 |
| Grade 8 | 699 | 839 |
Use the 75% level for joints that will be disassembled or re-torqued, and the 90% level only where the joint is permanent and the assembly method is controlled. These are percentages of the SAE J429 proof stress, not of the yield strength.
Inch ↔ Metric and SAE ↔ ISO
Order-of-magnitude comparison only, not equivalence — the two systems specify different diameters and thread forms, so torque values must not be carried from one column to the other.
| Inch (UNC) | Nearest metric |
|---|---|
| 1/4" | M6 |
| 5/16" | M8 |
| 3/8" | M10 |
| 7/16" | M12 |
| 1/2" | M12 |
| 9/16" | M14 |
| 5/8" | M16 |
| 3/4" | M20 |
| 7/8" | M22 |
| 1" | M24 |
| 1-1/8" | M30 |
| 1-1/4" | M33 |
| 1-1/2" | M36 |
| 2" | — |
SAE Grade ↔ ISO Property Class
| SAE J429 | Approx. ISO 898-1 |
|---|---|
| Grade 2 | 4.8 |
| Grade 5 | 8.8 |
| Grade 8 | 10.9 |
Approximate only: the proof stresses are not equal even where the strength bands line up (Grade 5 sits near ISO 8.8, but the two standards publish different proof values).
Basis of Calculation & Sources
T = K · d · F: K is the nut factor, d the nominal diameter and F the target preload = SAE J429 proof stress × tensile stress area × 75% (standard level) or 90% (yield level). The stress area follows At = π/4·(d − 0.9743/n)² (ASME B1.1), which for 7/8"-9 gives At = 298 mm². Proof stresses are the published SAE J429 values, which step down by nominal diameter tier. K follows published empirical ranges per friction state (Machinery's Handbook); the grease/MoS₂ band (K≈0.11) nearly overlaps anti-seize and is available in the calculator instead of a separate column.
Boundary: this is a simplified K-factor estimate, not a full VDI 2230 analysis (joint resilience, embedment and assembly method are not modelled). Friction is the dominant variable — no single torque value exists for a size without its friction state. Production joints follow the design specification or the manufacturer's torque table.
FAQ
What is the tightening torque for a 7/8"-9 Grade 5 bolt?
It depends on the friction state (typical values at 75% of proof stress): dry/black ≈ 582 N·m, electro zinc ≈ 495 N·m, hot-dip galvanized ≈ 641 N·m, oiled ≈ 379 N·m, anti-seize ≈ 291 N·m. Ranges are in the table above — no single torque value exists without its friction state.
Why do UNC and UNF of the same 7/8" take different torque?
Because the stress area differs: 7/8"-9 UNC has At = 298 mm² while 7/8"-14 UNF has At = 329 mm². A finer thread leaves more metal at the minor diameter, so the same grade can be preloaded higher. The UNF table above is calculated independently, not scaled from UNC.
What is the target preload for a 7/8" bolt?
At 75% of the SAE J429 proof stress: Grade 2 ≈ 50.9 kN, Grade 5 ≈ 131 kN, Grade 8 ≈ 184.9 kN. Proof stress steps down above each grade's diameter tier (Grade 2 above 3/4", Grade 5 above 1", Grade 8 above 1-1/2"), so preload does not scale linearly with grade on the larger sizes.
What basis are these torque values calculated on?
The K-factor method T = K·d·F, with F = 75% (or 90%) of the SAE J429 proof stress × tensile stress area, and At = π/4·(d − 0.9743/n)² per ASME B1.1. Estimated values, not assembly instructions — run a full VDI 2230 analysis for critical joints.
Related References
Bolt Torque Calculator
Interactive version: imperial and metric sizes, fine threads, stud bolts, custom K and a torque check.
M22 Bolt Torque Chart
The metric counterpart of this page: identical K-factor basis, six property classes across five friction states.
M22 Hex Bolt Weight Chart
Per-piece and per-1,000-pcs weights across the full length series of the nearest metric size.
M22 Stud Bolt Torque Chart
Torque and preload for stud bolts — threaded at both ends with no head to grip — where the tightening basis differs from a hex bolt.
Disclaimer: This chart is an engineering estimation for selection and budgeting only. It does not constitute assembly instructions or acceptance criteria.