ENGINEERING TOOL
M20 Bolt Torque Chart: Specs, Values & Settings
Class 8.8 · Target preload (kN) 109.8
| Dry / Black | 439 N·m 395–483 |
| Electro Zinc | 373 N·m 329–439 |
| Hot-Dip Galv. | 483 N·m 395–549 |
| Oiled | 285 N·m 263–329 |
M20 bolt torque spec: class 8.8 dry is 439 N·m (324 ft·lb). Values cover six property classes across five friction states — dry, zinc, HDG, oiled and anti-seize. K-factor method, basis fully disclosed.
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Torque by Property Class and Friction State
Quick answer: class 8.8 · dry / black ≈ 439 N·m (395–483); other classes and friction states below.
| Class | Rp0.2 (MPa) | Target 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 |
|---|---|---|---|---|---|---|---|
| 4.8 | 340 | 58.3 | 233 210–257 | 198 175–233 | 257 210–292 | 152 140–175 | 117 93–140 |
| 8.8 | 640 | 109.8 | 439 395–483 | 373 329–439 | 483 395–549 | 285 263–329 | 220 176–263 |
| 10.9 | 940 | 161.2 | 645 580–709 | 548 484–645 | 709 580–806 | 419 387–484 | 322 258–387 |
| 12.9 | 1100 | 188.7 | 755 679–830 | 641 566–755 | 830 679–943 | 490 453–566 | 377 302–453 |
| A2-70 | 450 | 77.2 | 309 278–340 | 262 232–309 | 340 278–386 | 201 185–232 | 154 123–185 |
| A4-80 | 600 | 102.9 | 412 370–453 | 350 309–412 | 453 370–515 | 268 247–309 | 206 165–247 |
Cell shows the typical value; the small line below is the range (min–max).
References
- ISO 898-1:2013 — Mechanical properties of fasteners made of carbon and alloy steel
- ISO 3506-1:2020 — Mechanical properties of corrosion-resistant stainless steel fasteners (A2/A4)
- GB/T 3098.1-2010 — Mechanical properties of bolts, screws and studs (CN national standard, official full text)
- GB/T 3098.6-2014 — Mechanical properties of stainless steel bolts, screws and studs (CN, official full text)
- ISO 16047:2005 — Fasteners: torque/clamp force testing (K-factor measurement method)
- Machinery's Handbook (30th ed.) — empirical K-factor ranges
- Machine Design — anti-seize compound friction test (K≈0.10)
Fine-Thread Torque Matrix
Fine-thread series (M20 × 1.5 mm) target preload and tightening torque ranges, same basis as the coarse chart; the fine-thread stress area As follows ISO 898-1:2013 Table 6 (this page takes M20 As = 272 mm²).
| Class | Rp0.2 (MPa) | Target 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 |
|---|---|---|---|---|---|---|---|
| 4.8 | 340 | 64.7 | 259 233–285 | 220 194–259 | 285 233–324 | 168 155–194 | 129 104–155 |
| 8.8 | 640 | 121.9 | 487 439–536 | 414 366–487 | 536 439–609 | 317 292–366 | 244 195–292 |
| 10.9 | 940 | 179 | 716 644–787 | 609 537–716 | 787 644–895 | 465 430–537 | 358 286–430 |
| 12.9 | 1100 | 209.4 | 838 754–922 | 712 628–838 | 922 754–1047 | 545 503–628 | 419 335–503 |
| A2-70 | 450 | 85.7 | 343 308–377 | 291 257–343 | 377 308–428 | 223 206–257 | 171 137–206 |
| A4-80 | 600 | 114.2 | 457 411–503 | 388 343–457 | 503 411–571 | 297 274–343 | 228 183–274 |
Cell shows the typical value; the small line below is the range (min–max).
Basis of Calculation & Sources
Calculation basis
- Torque follows the K-factor (nut factor) method T = K·D·F: D is the nominal diameter and F the target preload = 70% of Rp0.2 × thread stress area (M20 As = 245 mm²).
- Yield strengths are published values from ISO 898-1 (carbon/alloy steel) and ISO 3506-1 (stainless A2/A4).
- K follows published empirical ranges per friction condition (Machinery's Handbook; the wax/PTFE/anti-seize band K≈0.10 follows Machine Design test data).
- The grease/MoS₂ band (K≈0.11) nearly overlaps anti-seize and is available in the calculator instead of a separate column.
Stainless note: A2/A4 austenitic thread pairs gall easily under frictional heating. Always apply anti-seize and follow the Wax/PTFE/Anti-seize column (K≈0.10).
Scope & limits
- This chart is a simplified K-factor estimate, not a full VDI 2230 analysis (which accounts for joint resilience, embedment and assembly method).
- Friction is the dominant variable — no single torque value exists without its friction state.
- Always follow the design specification or manufacturer torque table.
Cite this chart
YAXIIO. M20 Bolt Torque Chart: Specs, Values & Settings. https://www.yaxiio.com/en/tools/bolt-torque-chart/m20 (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
FAQ
What is the target preload for an M20 bolt?
At 70% of Rp0.2: class 4.8 ≈ 58.3 kN, 8.8 ≈ 109.8 kN, 10.9 ≈ 161.2 kN; stainless A2-70 ≈ 77.2 kN and A4-80 ≈ 102.9 kN.
Can I use carbon-steel torque values for stainless M20 (A2-70/A4-80)?
No. The classes have different yield strengths (A2-70 = 450 MPa, A4-80 = 600 MPa), so target preloads differ; and stainless thread pairs gall easily — always apply anti-seize and follow the Wax/PTFE/Anti-seize column (K≈0.10).
Next: work out your order, or pick the size by standard
Related References
Bolt Torque Calculator
Interactive version: fine threads, imperial, stud bolts, custom K and torque check — this size is pre-filled.
M20 Hex Bolt Weight Chart
Per-piece and per-1,000-pcs weights across the full length series of the same size.
M20 Hex Key Size Chart
Which hex key (Allen wrench) fits a socket head cap screw of the same size: socket width across flats, key long and short arm lengths, and unit weight on the GB/T 70.1 + GB/T 5356 basis.
Tap Drill for the Thread Being Tightened
The hole under the bolt: rule tap drill value plus the nearest drill bit in the set, stated ahead of the torque tables.
Disclaimer: This chart is an engineering estimation for selection and budgeting only. It does not constitute assembly instructions or acceptance criteria.