Stainless steel fasteners require specific torque values — over-tightening causes galling (cold-welding), while under-tightening risks joint failure. This guide covers the key values and how to avoid common mistakes.
- Galling occurs when stainless threads seize under excessive torque — the threads cold-weld and the bolt snaps.
- Prevention: use anti-seize lubricant (Molykote or copper-based) and reduce torque by ~20%.
- Dry torque for A2-70: M6 = 9.5 Nm, M8 = 23 Nm, M10 = 46 Nm, M12 = 80 Nm.
- Always check manufacturer data for safety-critical applications.
What is galling?
Galling is a form of cold-welding that occurs when stainless steel surfaces under load seize together. As a bolt is tightened, the contact pressure between thread flanks can cause localised adhesion. Once the surfaces bond, continued tightening tears the material — the threads are destroyed and the bolt may snap entirely.
Galling is far more common in stainless than in carbon steel because stainless lacks the natural lubricity of carbon steel’s iron-oxide surface layer. The austenitic grades (A2, A4) are particularly prone.
Preventing galling
Two simple steps eliminate most galling risk:
- Use anti-seize lubricant — Molykote® 1000 (nickel-based) or a copper-based anti-seize paste on thread flanks and under the head.
- Reduce applied torque by ~20% from standard dry values when using a lubricant — lubrication increases the clamping force achieved at the same torque setting.
- Tighten steadily — rapid power-tool tightening generates heat and increases galling risk. Use a calibrated torque wrench.
- Check thread condition — contamination, damage or misalignment dramatically increases the risk.
Torque reference values — A2-70
The following values are for dry, uncoated A2-70 stainless fasteners. Reduce by ~20% when using lubricant. Always verify against the fastener manufacturer’s data and your joint design for safety-critical applications.
| Size | Pitch (mm) | Dry torque (Nm) | Lubricated torque (Nm) |
|---|---|---|---|
| M4 | 0.7 | 2.8 | 2.2 |
| M5 | 0.8 | 5.5 | 4.4 |
| M6 | 1.0 | 9.5 | 7.6 |
| M8 | 1.25 | 23 | 18 |
| M10 | 1.5 | 46 | 37 |
| M12 | 1.75 | 80 | 64 |
| M16 | 2.0 | 195 | 156 |
| M20 | 2.5 | 390 | 312 |
Frequently asked questions
Can I use standard carbon-steel torque values for stainless?
Does grade (A2 vs A4) affect torque values?
What anti-seize should I use with stainless fasteners?
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