Stainless steel bolts cannot be directly compared to carbon steel using the same torque tables. The key differences are lower proof strength (typically 70–210 ksi depending on grade), higher friction coefficients, and susceptibility to galling — a form of cold welding where stainless threads seize during tightening.
The most common stainless bolt grades are A2-70 (304 stainless) and A4-80 (316 stainless), designated under ISO 3506. These are NOT equivalent to carbon steel classes 8.8 or 10.9 — the "70" and "80" refer to minimum tensile strength in kgf/mm², which works out to roughly 700 MPa and 800 MPa respectively.
| Condition | K-Factor | Notes |
|---|---|---|
| Dry (unlubricated) | 0.30–0.45 | High and unpredictable — galling risk |
| Anti-seize compound | 0.12–0.15 | Strongly recommended for all stainless |
| Wax-coated (factory) | 0.10–0.12 | Best consistency |
| Silver-plated | 0.13–0.16 | Common in chemical/food processing |
Values assume K = 0.14 with anti-seize. Dry stainless torque is unreliable — always lubricate.
| Size | N·m | ft·lbs |
|---|---|---|
| M6 | 5.5 | 4.1 |
| M8 | 13.5 | 10.0 |
| M10 | 27 | 20 |
| M12 | 47 | 35 |
| M16 | 117 | 86 |
| M20 | 230 | 170 |
Always lubricate. Anti-seize compound (nickel or copper based) is essential on stainless fasteners. Dry assembly is the single most common cause of galling failures.
Slow down. High tightening speed generates friction heat, which accelerates galling. Use a hand wrench or low-speed power tool, never an impact gun, for final tightening.
Mix alloys when possible. Using a slightly different stainless alloy for the nut vs. bolt (e.g., 304 bolt with 316 nut) reduces galling tendency because the two surfaces have different hardness.
Don't reuse. Stainless bolts that have been tightened to near their proof load should not be reused — microscopic galling damage from the first installation makes seizure much more likely on reassembly.