The two most widely used metric property classes in industrial fastening. Here's how their torque specs compare across all common sizes.
Proof stress: 580 MPa · Tensile: 800 MPa
Material: Medium carbon steel, quenched & tempered
Use: General structural, automotive, machinery — the metric equivalent of SAE Grade 5
Proof stress: 830 MPa · Tensile: 1040 MPa
Material: Alloy steel, quenched & tempered
Use: High-strength joints, heavy equipment, flanges — the metric equivalent of SAE Grade 8
| Size | Class 8.8 | Class 10.9 | Difference |
|---|---|---|---|
| M6 | 10.5 N·m | 15.0 N·m | +43% |
| M8 | 25.5 N·m | 36.5 N·m | +43% |
| M10 | 50.5 N·m | 72.2 N·m | +43% |
| M12 | 88.0 N·m | 125.9 N·m | +43% |
| M14 | 140.6 N·m | 201.2 N·m | +43% |
| M16 | 218.1 N·m | 312.1 N·m | +43% |
| M18 | 301.4 N·m | 431.3 N·m | +43% |
| M20 | 425.9 N·m | 609.5 N·m | +43% |
| M22 | 580.7 N·m | 831.0 N·m | +43% |
| M24 | 736.0 N·m | 1053.3 N·m | +43% |
| M27 | 1079.2 N·m | 1544.3 N·m | +43% |
| M30 | 1463.1 N·m | 2093.8 N·m | +43% |
Class 8.8 is the standard for most industrial and automotive applications. It's the default choice when an engineer specifies a "high-strength bolt" without further qualification. Widely stocked, moderately priced, and suitable for the vast majority of bolted joints that see static or moderate dynamic loads.
Class 10.9 provides roughly 43% more clamping force at the same bolt size. This matters in space-constrained designs where stepping up to a larger bolt diameter isn't an option. Common in heavy machinery, mining equipment, wind turbine towers, and pressure vessel flanges where the bolted joint is the critical structural element.
Cost vs. performance: Class 10.9 bolts typically cost 20–40% more than 8.8 and require matched nuts (class 10 or higher). In many cases, an M10 class 10.9 bolt can replace an M12 class 8.8 bolt, saving weight and space despite the per-unit cost increase.