The coupling nut must develop the full strength of the threaded fasteners that it is joining together. Coupling nuts are graded the same way standard hex nuts are graded. They must meet specific ASTM and SAE chemical and mechanical requirements to be compatible with the various grades of fasteners they are used with. Dimensional requirements are covered under ASME B18.2.2
The chart below specifies the grades of coupling nuts to be used with various grades of bolts, thread rods, and other fasteners. Some stainless steel coupling nuts are ordered to a raw material grade (Type 304 or 316) as opposed to an ASTM nut specification.
| Grade of Bolt | Surface Finish | Nominal Size Inches | A563 Grade and ANSI Nut Style | |||
|---|---|---|---|---|---|---|
| Recommended | Suitable | |||||
| Hex | Heavy Hex |
Hex | Heavy Hex |
|||
| Any | 1⁄4 to 11⁄2 |
A
|
B,D,DH
|
A,B,C,D,DH,DH3
|
||
| >11⁄2 to 2 |
A
|
A
|
C,D,DH,DH3
|
|||
| >2 to 4 |
A
|
C,D,DH,DH3
|
||||
| Any | 1⁄4 to 11⁄2 |
A
|
B,D,DH
|
A,B,C,D,DH,DH3
|
||
| >11⁄2 to 2 |
A
|
A
|
||||
| >2 to 4 |
A
|
|||||
| Plain | 1⁄2 – 11⁄2 |
C
|
C3,D,DH,DH3
|
|||
| Galvanized | 1⁄2 – 11⁄2 |
DH
|
||||
| Plain | 1⁄2 – 11⁄2 |
C3
|
DH3
|
|||
| Plain | 1⁄4 to 11⁄2 |
C
|
D,DH
|
C3,D,DH,DH3
|
||
| >11⁄2 to 4 |
C
|
C3,D,DH,DH3
|
||||
| Galvanized | 1⁄4 to 11⁄2 |
DH
|
||||
| >11⁄2 to 4 |
DH
|
|||||
| Plain | 1⁄4 to 11⁄2 |
DH
|
DH
|
D,DH,DH3
|
||
| >11⁄2 to 4 |
DH
|
DH3
|
||||
| Plain | 1⁄4 to 11⁄2 |
B
|
D,DH
|
B,C,C3,D,DH,DH3
|
||
| >11⁄2 to 3 |
A
|
C,C3,D,DH,DH3
|
||||
| Galvanized | 1⁄4 to 11⁄2 |
DH
|
D,DH
|
D
|
||
| >11⁄2 to 3 |
DH
|
D
|
||||
| Plain | 1⁄2 – 11⁄2 |
DH
|
DH3
|
|||
| Plain | 1⁄2 – 11⁄2 |
DH3
|
||||
|
A687
|
Plain | 5⁄8 – 3 |
D
|
DH,DH3
|
||
| Galvanized | 5⁄8 – 3 |
DH
|
||||
| Any | 1⁄2 to 11⁄2 |
A
|
B,D,DH
|
A,B,C,D,DH,DH3
|
||
| >11⁄2 to 4 |
A
|
C,D,DH,DH3
|
||||
| Plain | 1⁄2 to 11⁄2 |
A
|
B,D,DH
|
A,B,C,D,DH,DH3
|
||
| >11⁄2 to 4 |
A
|
C,D,DH,DH3
|
||||
| Galvanized | 1⁄2 to 4 |
A
|
C,D,DH,DH3
|
|||
| Plain | 1⁄2 to 11⁄2 |
D
|
DH,DH3
|
|||
| >11⁄2 to 3 |
DH
|
DH3
|
||||
| Galvanized | 1⁄2 to 3 |
DH
|
DH3
|
|||
| 1. Nuts made in accordance with the requirements of Specification A194 Grade 2H and marked with its grade symbol are an acceptable equivalent for Grade DH nuts. (per ASTM A563/A563M-21a, Table X1.1) | ||||||