Matching Nut Grades to Bolt Grades
Using the wrong nut with a high-strength bolt is a surprisingly common mistake — and it’s dangerous. A Grade 5 nut on a Grade 8 bolt will strip the nut threads before the bolt reaches its rated clamp load. Always match (or exceed) the nut grade to the bolt grade.
Quick Matching Guide
| Bolt Grade | Minimum Nut Grade |
|---|---|
| Grade 2 | Grade 2 |
| Grade 5 | Grade 5 |
| Grade 8 | Grade 8 (or Grade C) |
| A325 | A563 Grade DH |
| A490 | A563 Grade DH3 |
| Metric 8.8 | Class 8 |
| Metric 10.9 | Class 10 |
| Metric 12.9 | Class 12 |
Nut Grade Markings & Properties
How strong are these nuts? That’s what she said. Seriously, what do those marks mean? Should I order a hex nut with one dot or three dashes? Here’s a cheat sheet with nut identifying marks, material grade and mechanical properties.
| Grade Identification Marking |
Specification | Material | Nominal Size In. |
Proof Load Stress ksi |
Hardness Rockwell |
See Note |
|
| Min | Max | ||||||
 No Mark |
ASTM A563 – Grade 0 | Carbon Steel | 1/4 thru 1-1/2 | 69 | B55 | C32 | 3,4 |
| ASTM A563 – Grade A | Carbon Steel | 1/4 thru 1-1/2 | 90 | B68 | C32 | 3,4 | |
| ASTM A563 – Grade B | Carbon Steel | 1/4 thru 1 | 120 | B69 | C32 | 3,4 | |
| over 1 thru 1-1/2 | 105 | ||||||
 |
ASTM A563 – Grade C | Carbon Steel May be Quenched and Tampered |
1/4 thru 4 | 144 | B78 | C38 | 5 |
 |
ASTM A563 – Grade C3 | Atmospheric Corrosion Resistant Steel May be Quenched and Tampered |
1/4 thru 4 | 144 | B78 | C38 | 5,9 |
 |
ASTM A563 – Grade D | Carbon Steel May be Quenched and Tampered |
1/4 thru 4 | 150 | B84 | C38 | 6 |
 |
ASTM A563 – Grade DH | Carbon Steel Quenched and Tampered |
1/4 thru 4 | 175 | C24 | C38 | 6 |
 |
ASTM A563 – Grade DH3 | Atmospheric Corrosion Resistant Steel, Quenched and Tampered |
1/4 thru 4 | 175 | C24 | C38 | 5,9 |
 |
ASTM A194 – Grade 1 | Carbon Steel | 1/4 thru 4 | 130 | B70 | — | 7 |
 |
ASTM A194 – Grade 2 | Medium Carbon Steel |
1/4 thru 4 | 150 | 159 | 352 | 7,8 |
 |
ASTM A194 – Grade 2H | Medium Carbon Steel, Quenched and Tempered |
1/4 thru 4 | 175 | C24 | C38 | 7 |
 |
ASTM A194 – Grade 2HM | Medium Carbon Steel, Quenched and Tempered |
1/4 thru 4 | 150 | 159 | 237 | 7,8 |
 |
ASTM A194 – Grade 4 | Medium Carbon Alloy Steel, Quenched and Tempered |
1/4 thru 4 | 175 | C24 | C38 | 7 |
 |
ASTM A194 – Grade 7 | Medium Carbon Alloy Steel, Quenched and Tempered |
1/4 thru 4 | 175 | C24 | C38 | 7 |
 |
ASTM A194 – Grade 7M | Medium Carbon Alloy Steel, Quenched and Tempered |
1/4 thru 4 | 150 | 159 | 237 | 7 |
| See Note 1,2 | 10 | ||||||