Matching-nut guide
Rule of thumb: the nut must be at least as strong as the bolt it retains. Under-matched nuts strip before the bolt reaches its rated load. For stainless, always match the alloy (A2 with A2, A4 with A4) to avoid galling and galvanic issues.
Metric
| Bolt grade / class | Recommended nut | Notes |
|---|---|---|
| Class 8.8 | ISO 898-2 Class 8 (or 10) | Standard structural pairing. |
| Class 10.9 | ISO 898-2 Class 10 | Match class for full joint strength. |
| Class 12.9 | ISO 898-2 Class 12 | High-strength critical joints. |
| Class 4.6 / 4.8 / 5.8 | ISO 898-2 Class 5 (min 4) | General / low-duty. |
Metric stainless
| Bolt grade / class | Recommended nut | Notes |
|---|---|---|
| A2-70 / A4-70 | Matching A2 / A4 nut | Same alloy to avoid galling & galvanic issues. |
Imperial
| Bolt grade / class | Recommended nut | Notes |
|---|---|---|
| SAE Grade 2 | SAE J995 Grade 2 (no marks) | General purpose. |
| SAE Grade 5 | SAE J995 Grade 5 (or ASTM A563 A/B) | Automotive / machinery. |
| SAE Grade 8 | SAE J995 Grade 8 (or ASTM A563 C/DH) | Heavy / high-strength. |
| ASTM A325 | ASTM A563 Grade C / DH | Structural steel connections. |
| ASTM A490 | ASTM A563 Grade DH | High-strength structural. |
| ASTM A193 B7 | ASTM A194 Grade 2H | High-temp / high-pressure flanges. |
Per ISO 898-2 (metric nuts), ISO 3506-2 (stainless), SAE J995 and ASTM A563 / A194. Verify against your engineering spec for critical joints.