Fastening Know-How
ASTM A563 Nuts: Grades, Proof Loads and Bolt Pairing
ASTM A563 is the US standard for carbon and alloy steel nuts. Its grades run from O, A and B for general bolting up to C, D, DH and the weathering grades C3 and DH3, which are the heavy hex nuts used with ASTM F3125 structural bolts.
What ASTM A563 Covers
A563 sets chemical and mechanical requirements for carbon and alloy steel nuts used on bolts, studs and other externally threaded parts. The grade tells you the proof load the nut must carry without stripping, its hardness range and how it is marked.
Grades with a “3” (C3 and DH3) are made from atmospheric corrosion resistant, or weathering, steel to match Type 3 structural bolts.
CSG supplies A563 nuts in its nuts and structural fasteners ranges. For SAE graded bolts, see Grade 8 bolts.
ASTM A563 Grade Chart
Proof load stress, hardness and marking by grade.
| Grade | Nut style | Size range (in.) | Proof load stress (ksi) | Hardness, Rockwell | Material | Grade marking |
|---|---|---|---|---|---|---|
| O | Hex | 1/4 to 1-1/2 | 69 | B55 to C32 | Carbon steel | Not required; "O" if marked |
| A | Hex | 1/4 to 1-1/2 | 90 | B68 to C32 | Carbon steel | Not required; "A" if marked |
| B | Hex | 1/4 to 1 / over 1 to 1-1/2 | 120 / 105 | B69 to C32 | Carbon steel | Not required; "B" if marked |
| C | Heavy hex | 1/4 to 4 | 144 | B78 to C38 | Carbon steel, may be quenched and tempered | Three arcs |
| C3 | Heavy hex | 1/4 to 4 | 144 | B78 to C38 | Weathering steel, may be quenched and tempered | Three arcs with a "3" |
| D | Heavy hex | 1/4 to 4 | 150 | B84 to C38 | Carbon steel, may be quenched and tempered | D |
| DH | Heavy hex | 1/4 to 4 | 175 | C24 to C38 | Carbon steel, quenched and tempered | DH |
| DH3 | Heavy hex | 1/4 to 4 | 175 | C24 to C38 | Weathering steel, quenched and tempered | DH3 |
Sources, matched value by value: American Fastener Technologies, ASTM, SAE and ISO grade markings chart, and American Threaded Products, ASTM A563 specifications (both accessed 2 Oct 2026). Grade C, C3, D, DH and DH3 markings also confirmed against Figure C-2.1 of the RCSC Specification for Structural Joints Using High-Strength Bolts (2020). Values for Grades O, A and B are for plain hex nuts. Heavy hex values for Grades A and B, and galvanized proof loads, are withheld because only one source lists them. Check the current edition of ASTM A563 before relying on this for design.
Grades C, C3, D, DH and DH3 must also carry the manufacturer identification mark. Grades O, A and B are only marked when the order asks for it.
Which A563 Nut Grade to Use With F3125 Bolts
For pretensioned and snug-tight structural joints, the RCSC specification limits which nut grades may be used with each bolt group. Bolt grades are explained on the ASTM F3125 page.
| Bolt | Bolt type | Finish | Permitted A563 nut grades | Washer |
|---|---|---|---|---|
| F3125 Grade A325 or F1852 (120 ksi) | Type 1 | Plain | C, C3, D, DH or DH3 | F436 Type 1 |
| F3125 Grade A325 or F1852 (120 ksi) | Type 1 | Galvanized or coated | DH | F436 Type 1 |
| F3125 Grade A325 or F1852 (120 ksi) | Type 3 | Plain | C3 or DH3 | F436 Type 3 |
| F3125 Grade A490 or F2280 (150 ksi) | Type 1 | Plain | DH or DH3 | F436 Type 1 |
| F3125 Grade A490 or F2280 (150 ksi) | Type 1 | Approved coating | DH | F436 Type 1 |
| F3125 Grade A490 or F2280 (150 ksi) | Type 3 | Plain | DH3 | F436 Type 3 |
Sources: RCSC Specification for Structural Joints Using High-Strength Bolts (2020), Table 2.2 and Section 2.5, and Portland Bolt A325 and F3125 hardware pages (accessed 2 Oct 2026). The coated A490 row is stated by RCSC only; Portland Bolt does not list it. ASTM A194 Grade 2H nuts are permitted as a substitute for A563 Grade DH (RCSC 2.3.2; Portland Bolt).
Finishes and Overtapping
A563 nuts are supplied plain or with the same coating as the bolt. Hot-dip galvanized (ASTM F2329) and mechanically galvanized (ASTM B695 Class 55) finishes are used with galvanized A325 bolts, and every threaded part of the assembly should be galvanized by the same process.
Because zinc builds up on the bolt thread, galvanized nuts are tapped oversize, called overtapping, so the assembly still fits. For coated A325 assemblies, the RCSC specification limits the nut to Grade DH.
A490 bolts are not hot-dip or mechanically galvanized, so their nuts are normally plain or carry an approved zinc-flake coating. Read more in A325 vs A490 bolts.
ASTM A563 Nut FAQ
What is an ASTM A563 Grade DH nut?
A quenched and tempered carbon steel heavy hex nut with a proof load stress of 175 ksi and a hardness of C24 to C38. It is the usual nut for F3125 Grade A325 and A490 bolts.
Can I use an A194 2H nut instead of A563 DH?
Yes. The RCSC specification permits ASTM A194 Grade 2H nuts as a substitute for A563 Grade DH on structural bolts.
Which nut goes with a galvanized A325 bolt?
An A563 Grade DH nut, galvanized by the same process as the bolt and overtapped to fit the coating.
What does the 3 in C3 and DH3 mean?
It marks a weathering-steel nut, made to match Type 3 weathering-steel structural bolts.
Are A563 nuts always marked?
Grades C, C3, D, DH and DH3 carry a grade mark and the manufacturer mark. Grades O, A and B are only marked when the purchaser asks for it.
Matching Nuts to Structural Bolts?
Send the bolt grade, size, finish and quantity. We supply heavy hex nuts, bolts and F436 washers as matched sets with certification.