ENGINEERING TOOL
M8 Bolt Torque Chart
Target preload and tightening torque ranges (min–typ–max) for M8 bolts across six property classes and five friction states. K-factor method, basis fully disclosed.
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Torque by Property Class and Friction State
Quick answer: class 8.8 · electro zinc ≈ 22 N·m (20–26); 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 | 8.7 | 14 13–15 | 12 10–14 | 15 13–17 | 9 8–10 | 7 6–8 |
| 8.8 | 640 | 16.4 | 26 24–29 | 22 20–26 | 29 24–33 | 17 16–20 | 13 10–16 |
| 10.9 | 940 | 24.1 | 39 35–42 | 33 29–39 | 42 35–48 | 25 23–29 | 19 15–23 |
| 12.9 | 1100 | 28.2 | 45 41–50 | 38 34–45 | 50 41–56 | 29 27–34 | 23 18–27 |
| A2-70 | 450 | 11.5 | 18 17–20 | 16 14–18 | 20 17–23 | 12 11–14 | 9 7–11 |
| A4-80 | 600 | 15.4 | 25 22–27 | 21 18–25 | 27 22–31 | 16 15–18 | 12 10–15 |
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 (M8 × 1 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 M8 As = 39.2 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 | 9.3 | 15 13–16 | 13 11–15 | 16 13–19 | 10 9–11 | 7 6–9 |
| 8.8 | 640 | 17.6 | 28 25–31 | 24 21–28 | 31 25–35 | 18 17–21 | 14 11–17 |
| 10.9 | 940 | 25.8 | 41 37–45 | 35 31–41 | 45 37–52 | 27 25–31 | 21 17–25 |
| 12.9 | 1100 | 30.2 | 48 43–53 | 41 36–48 | 53 43–60 | 31 29–36 | 24 19–29 |
| A2-70 | 450 | 12.3 | 20 18–22 | 17 15–20 | 22 18–25 | 13 12–15 | 10 8–12 |
| A4-80 | 600 | 16.5 | 26 24–29 | 22 20–26 | 29 24–33 | 17 16–20 | 13 11–16 |
Cell shows the typical value; the small line below is the range (min–max).
Basis of Calculation & Sources
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 (M8 As = 36.6 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).
Boundary: 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.
Corresponding standard page
Open the GB/T 5783 / DIN 933 standard product page: sizes, materials and finishes, and ordering questions.
Open the GB/T 5783 / DIN 933 page
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FAQ
What is the tightening torque for an M8 class 8.8 bolt?
It depends on the friction state (typical values at 70% of Rp0.2 target preload): oiled ≈ 17 N·m, electro zinc ≈ 22 N·m, hot-dip galvanized ≈ 29 N·m, dry/black ≈ 26 N·m, anti-seize ≈ 13 N·m. See the matrix above for ranges — no single torque value exists without its friction state.
What is the target preload for an M8 bolt?
At 70% of Rp0.2: class 4.8 ≈ 8.7 kN, 8.8 ≈ 16.4 kN, 10.9 ≈ 24.1 kN; stainless A2-70 ≈ 11.5 kN and A4-80 ≈ 15.4 kN.
Can I use carbon-steel torque values for stainless M8 (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).
What basis are these torque values calculated on?
The K-factor method T = K·D·F with target preload = 70% of Rp0.2 × stress area (yield strengths per ISO 898-1 / ISO 3506-1 published values) and K per published empirical friction ranges. Estimated values, not assembly instructions; run a full VDI 2230 analysis for critical joints.
Related References
Bolt Torque Calculator
Interactive version: fine threads, imperial, stud bolts, custom K and torque check — this size is pre-filled.
Metric Bolt Torque Chart (Reference)
Full M6–M36 tables with disclosed formula and GB/ISO clause citations; every value reproducible.
M8 Hex Bolt Weight Chart
Per-piece and per-1,000-pcs weights across the full length series of the same size.
M8 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.
Disclaimer: This chart is an engineering estimation for selection and budgeting only. It does not constitute assembly instructions or acceptance criteria.