Fastening Know-How
ASTM F3125 Structural Bolts: Grades A325, A490, F1852 and F2280
ASTM F3125 is the standard for high-strength heavy hex structural bolts and twist-off tension control bolt assemblies. It replaced six separate specifications, A325, A325M, A490, A490M, F1852 and F2280, which now live on as F3125 grades.
What ASTM F3125 Replaced
F3125 was first published as ASTM F3125-15 and brought six structural bolt standards under one specification. The old A325 and A490 standards were withdrawn by ASTM in 2016, so a current order or drawing should call for “ASTM F3125 Grade A325” rather than plain “A325”.
Inside F3125, Grade names the product (A325, A490 and so on), Type names the steel (Type 1 standard, Type 3 weathering), and Style separates heavy hex bolts from twist-off assemblies.
CSG supplies F3125 bolts with matching nuts and washers in its structural fasteners range. For bolt grade markings across SAE, ASTM and metric, see the bolt grade chart.
F3125 Grades at a Glance
Strength groups follow the RCSC specification: Group 120 and Group 150.
| F3125 grade | Style | RCSC group | Tensile strength, min | Tensile strength, max | Types |
|---|---|---|---|---|---|
| A325 | Heavy hex | 120 | 120 ksi | Withheld | 1 or 3 |
| A325M | Heavy hex, metric | 120 | 830 MPa | Withheld | 1 or 3 |
| F1852 | Twist-off tension control | 120 | 120 ksi | Withheld | 1 or 3 |
| A490 | Heavy hex | 150 | 150 ksi | 173 ksi | 1 or 3 |
| A490M | Heavy hex, metric | 150 | 1040 MPa | Withheld | 1 or 3 |
| F2280 | Twist-off tension control | 150 | 150 ksi | Withheld | 1 or 3 |
Sources: RCSC Specification for Structural Joints Using High-Strength Bolts (2020), Table 2.1 and Section 2 commentary; Portland Bolt, F3125 grades and specification pages; STRUCTURE magazine, "New Twists and Turns in Structural Bolting" (Nov 2015) (all accessed 2 Oct 2026). The F2280 maximum is withheld: only RCSC states it. This is a summary; check the current edition of ASTM F3125 before relying on it for design.
A325 Is Now 120 ksi at Every Size
The old A325 standard dropped its minimum tensile strength from 120 ksi to 105 ksi above 1 inch. F3125 Grade A325 removed that step, so the bolt is 120 ksi from 1/2 to 1-1/2 inch, and the twist-off F1852 grade changed the same way.
| Diameter | Old ASTM A325, min tensile | F3125 Grade A325, min tensile |
|---|---|---|
| 1/2" to 1" | 120 ksi | 120 ksi |
| 1-1/8" to 1-1/2" | 105 ksi | 120 ksi |
Sources: STRUCTURE magazine (Nov 2015) and Portland Bolt A325 properties page, which compares the old A325 with F3125 Grade A325; the RCSC specification (2020) states a 120 ksi minimum for Grades A325 and F1852 (accessed 2 Oct 2026).
Higher-strength larger bolts also mean higher minimum pretension values for those sizes, so check installation tables against the current specification.
Sizes, Types and Head Markings
F3125 heavy hex structural bolts run from 1/2 to 1-1/2 inch diameter. The twist-off grades F1852 and F2280 were limited to 1-1/8 inch under their old standards and now run to 1-1/4 inch.
Type 1 bolts are medium carbon, carbon boron or alloy steel. Type 3 bolts are weathering steel for exposed steelwork. If the type is not specified, either may be supplied.
Head markings did not change with F3125: the head carries the grade, such as A325 or A490, and the manufacturer identification mark. Bolts with non-standard heads or thread lengths made under supplementary requirement S2 are marked A325S or A490S.
Sources: Portland Bolt F3125 specification and grades pages; STRUCTURE magazine (Nov 2015); RCSC specification (2020), Figure C-2.1 and commentary (accessed 2 Oct 2026).
Matching Nuts and Washers
Nut grades come from ASTM A563 and washers from ASTM F436, with the washer type matching the bolt type. Full nut details are on the ASTM A563 nuts page.
| Bolt group | Bolt type and finish | A563 nut grade | Washer |
|---|---|---|---|
| 120 (A325, F1852) | Type 1, plain | C, C3, D, DH or DH3 | F436 Type 1 |
| 120 (A325, F1852) | Type 1, galvanized | DH | F436 Type 1 |
| 120 (A325, F1852) | Type 3, plain | C3 or DH3 | F436 Type 3 |
| 150 (A490, F2280) | Type 1, plain | DH or DH3 | F436 Type 1 |
| 150 (A490, F2280) | Type 3, plain | DH3 | F436 Type 3 |
Sources: RCSC specification (2020), Table 2.2 and Section 2.5, and Portland Bolt A325 and F3125 hardware pages (accessed 2 Oct 2026). ASTM A194 Grade 2H nuts may substitute for A563 Grade DH.
Galvanizing and Coatings
Grade A325 bolts may be hot-dip galvanized (ASTM F2329) or mechanically galvanized (ASTM B695 Class 55), with nut and bolt coated by the same process. Grade A490 and F2280 bolts must not be hot-dip or mechanically galvanized, because their strength puts them at risk of hydrogen embrittlement.
For 150 ksi bolts the qualified route is a zinc/aluminium flake coating applied under the specification. Twist-off assemblies are not hot-dip galvanized, and any coating on them is applied under the direction of the manufacturer.
Grade 8 bolts are not a substitute for F3125 in structural joints. See Grade 8 bolts for how they compare, and A325, A490, F3125, B7 and SAE grades compared for the wider picture.
ASTM F3125 FAQ
Is ASTM A325 still a valid specification?
A325 was withdrawn by ASTM in 2016 and replaced by ASTM F3125 Grade A325. The bolt and its head marking stayed the same, so “A325” now refers to the F3125 grade.
What is the difference between F3125 Grade A325 and A490?
A325 has a 120 ksi minimum tensile strength and may be galvanized. A490 has a 150 ksi minimum and 173 ksi maximum and must not be hot-dip or mechanically galvanized.
What are F1852 and F2280?
They are the twist-off tension control grades of F3125. F1852 matches A325 strength and F2280 matches A490 strength.
What does A325S mean on a bolt head?
It marks an F3125 Grade A325 bolt made under supplementary requirement S2, which allows non-standard heads or thread lengths, such as a fully threaded bolt.
Which nut should I use with an F3125 A325 bolt?
For a plain Type 1 bolt, A563 Grade C, C3, D, DH or DH3; for a galvanized bolt, Grade DH. See ASTM A563 nuts.
Supplying Structural Bolt Assemblies?
Send the grade, type, finish, sizes and quantities. We supply F3125 bolts with matching A563 nuts and F436 washers, with certification.