When A325 Isn't Enough: The Spec Guide to ASTM A490 Heavy-Hex Bolts
ASTM A490 is the high-strength structural bolt you specify when A325 doesn't close the gap. At 150 ksi minimum tensile strength — 25% stronger than A325's 120 ksi — it's the standard choice for heavy steel frames, long-span bridges, wind towers, and anywhere the connection geometry forces you toward a smaller bolt at higher load.
What trips people up isn't specifying A490. It's the coating restrictions. A490 has hard rules about what you can and cannot put on it, and the most common coating in structural work — hot-dip galvanizing — is explicitly prohibited. Getting that wrong in the field creates delays, rework, and occasionally failed connections.
This post covers the spec, mechanical properties, coating rules, and how A490 fits within the broader F3125 family.
A490 and F3125 — How the naming works
A490 was once a standalone ASTM standard. In 2015, it was consolidated into ASTM F3125 along with A325, F1852, and F2280. A490 is now formally designated F3125 Grade A490, but the familiar name stayed on the bolt head, and it stayed in the drawings. If you're reading a structural spec from any point in the last decade, "A490" means F3125 Grade A490.
The four original grades within F3125 are A325 (heavy hex, 120 ksi, 1/2" through 1-1/2"), A490 (heavy hex, 150–173 ksi, 1/2" through 1-1/2"), F1852 (twist-off, 120 ksi, up to 1-1/4"), and F2280 (twist-off, 150 ksi, up to 1-1/4"). The 2025 revision added Grade 144 — a 144 ksi grade available in both heavy hex and twist-off styles up to 1-1/4", bridging the gap between A325 and A490. If your project spec references Grade 144, confirm that the 2025 revision is in effect before fabrication.
Mechanical properties
Grade A490, Type 1: minimum tensile 150 ksi, maximum tensile 173 ksi, minimum yield (0.2% offset) 130 ksi, minimum elongation in 2" of 14%, minimum reduction of area 40%, proof load 120 ksi.
The maximum tensile matters — A490 has a ceiling as well as a floor. Bolts that come in above 173 ksi don't comply, even though they're stronger. The upper limit protects against embrittlement and ensures ductility at the connection.
A325 for comparison: 120 ksi tensile min, 92 ksi yield min, all diameters 1/2" through 1-1/2". The old A325 standard dropped from 120 to 105 ksi above 1" diameter — F3125 eliminated that drop. A325 is now uniform across the full size range.
Types
Type 1 is alloy steel, quenched and tempered. The standard choice for most applications.
Type 3 is weathering steel — the alloy itself provides atmospheric corrosion resistance, similar to A242 or A588 structural steel. Type 3 bolts are used with weathering-steel frames, where the connection is designed to be left unpainted. They are marked with an underline or identifier under the grade mark to distinguish them from Type 1 in the field.
Nuts and washers
For A490 (and F2280 twist-off), the specified nuts are A563 Grade DH or DH3, type matching the bolt. A194 Grade 2H is a permitted substitute for A563 DH in Type 1 applications. F436 hardened washers are the spec pairing and are required at slotted holes, oversized holes, and wherever turn-of-nut tensioning distributes load.
Don't shortcut the nut grade here. A563 DH and A194 2H are not interchangeable with lighter nut grades — the proof load requirements at 150 ksi bolt tension are not trivial.
Coatings — what F3125 actually says
This is where most ordering mistakes happen, so we'll be direct.
Hot-dip galvanizing is prohibited on A490. F3125 does not say "use with caution" or "requires special approval." It prohibits HDG on Grade A490 outright. The reason is hydrogen embrittlement: acid pickling and exposure to molten zinc in the HDG process introduce hydrogen into the steel, and at a tensile strength of 150 ksi, A490 is susceptible to delayed brittle fracture. The bolt can look fine, pass initial inspection, and fail under load weeks later.
Zinc electroplating is also prohibited on A490 for the same reason — the electrolytic bath introduces hydrogen that the plating process doesn't adequately bake out at this strength level.
What is approved for A490: zinc-flake coatings per ASTM F1136 Grade 3 or ASTM F2833 Grade 1, and mechanical galvanizing per ASTM B695.
Zinc-flake is applied by spray or dip-spin and oven cured — no acid pickling, no electrolytic deposition. Common commercial systems include Geomet and Magni. If a drawing calls out "Geomet or equivalent" on A490 bolts, it's invoking this coating family. Zinc-flake provides HDG-equivalent or better corrosion protection in a much thinner coating, with no HE risk.
Mechanical galvanizing is the cold-process alternative to HDG — zinc powder is mechanically tumbled onto parts, with no heat or acid. Same logic: avoids the acid pickling and thermal steps that introduce hydrogen, and is approved for A490.
If you're specifying A490 in an exterior or corrosive environment and your standard order template defaults to hot-dip galvanizing, catch that before the order hits the shop. It's a common mismatch.
Installation
F3125 bolts are pretensioned by one of four RCSC-defined methods: turn-of-nut (snug tight, then the specified additional rotation), calibrated wrench (torque to a value calibrated per lot on a Skidmore-Wilhelm device), twist-off tension control via F2280 (the splined end shears at calibrated tension), or direct tension indicator via F959 (a compressible washer where gap measurement confirms tension).
All four methods are valid for A490. DTIs are common when you need visible verification. The bolt-nut-washer assembly must maintain its rotational capacity through installation — don't clean off field lubricant supplied with the assembly.
A490 vs A325 — when to use which
A325 handles the majority of structural steel connections. Move to A490 when connection geometry limits bolt quantity or diameter, and you need more tension per bolt, or when the design load puts A325 at or near its pretension limit, or when the engineer has explicitly called A490.
Don't substitute A325 for A490 without engineering approval, even though it seems like a conservative swap — the connection was likely designed to the A490 pretension value, and swapping down changes the joint behavior. Equally, don't default to A490 to "be safe." It brings stricter coating constraints and higher costs, and if it gets galvanized by mistake, you have a compliance problem.
What California Fastener supplies
We stock F3125 Grade A490 in Type 1 and Type 3, with matching A563 DH heavy hex nuts and F436 washers. Mill certs ship with every order — heat number, mechanical properties, and rotational capacity test results for coated assemblies. For projects requiring submittal packages, we build those out to AISC fabrication standards.
When coating is required, we source and document the approved systems: zinc-flake per F1136 Grade 3 or F2833 Grade 1, and mechanical galvanizing per B695. We don't substitute HDG on A490 regardless of what the default coating line says on an order form — if that comes up in your supply chain, it's worth a conversation before it becomes a field problem.
Contact California Fastener to discuss A490 quantities, lead times, or coating requirements.
