On a structural steel job, two heavy-hex bolts can look almost identical sitting in a bolt bucket. Same diameter. Same general shape. Same black finish.
But one may be an ASTM F3125 Grade A325 and the other an ASTM F3125 Grade A490—and that difference matters.
The markings forged or stamped into the bolt head are there for a reason. They allow ironworkers, inspectors, fabricators, erectors, and quality-control personnel to identify what they are actually installing.
Learning to read those markings is one of those small pieces of trade knowledge that becomes extremely useful in the field.
This guide explains A325 vs. A490 structural bolts, bolt head markings, Type 1 and Type 3 bolts, strength differences, A325T markings, matching nuts and washers, and common identification mistakes.
First: A325 and A490 Are Now Grades Under ASTM F3125
One important detail gets missed surprisingly often.
People still commonly call them “A325 bolts” and “A490 bolts.” That terminology remains normal in the field, but technically the current structural-bolting specification is ASTM F3125/F3125M.
ASTM F3125 consolidated several older structural fastener specifications, including A325, A490, F1852, and F2280. Today, A325 and A490 continue as grades within F3125 rather than standalone specifications. (ASTM Store)
So the more complete terminology is:
ASTM F3125 Grade A325
and
ASTM F3125 Grade A490
That distinction is worth knowing when reading specifications, drawings, material certifications, purchase orders, inspection documents, or bolt installation procedures.
For workers moving between different crafts and industrial projects, Næxon also maintains a broader guide to the highest-paying skilled trades in the oil and gas industry, including ironworkers, boilermakers, millwrights, pipefitters, welders, and other industrial trades.
A325 vs. A490: The Basic Difference
The easiest starting point is strength.
ASTM F3125 Grade A325
Grade A325 structural bolts have a 120 ksi minimum tensile strength.
They are extremely common structural fasteners used in steel construction.
ASTM F3125 Grade A490
Grade A490 structural bolts have a 150 ksi minimum tensile strength.
That makes A490 a higher-strength structural bolt than A325. (ASTM Store)
A simple field memory aid is:
A325 = 120 ksi minimum tensile strength
A490 = 150 ksi minimum tensile strength
But that does not mean an ironworker can simply substitute an A490 whenever an A325 is specified.
Bolt grade is an engineering requirement.
The connection was designed around a particular bolt grade, diameter, hole condition, connection type, pretension requirement, installation method, and other factors.
Higher strength does not automatically mean an acceptable substitute.
Install what the drawings, specifications, approved bolt schedule, or engineering documents require.
Quick A325 vs. A490 Comparison
Feature
F3125 Grade A325
F3125 Grade A490
Minimum tensile strength
120 ksi
150 ksi
Relative strength
High strength
Higher strength
Common style
Heavy hex
Heavy hex
Typical head identification
A325
A490
Structural use
Yes
Yes
Type 3 available
Yes
Yes
Standard hot-dip galvanizing
Permitted under applicable requirements
Major restrictions apply
Substitution
Only when approved
Only when approved
ASTM F3125 currently covers heavy-hex A325 and A490 bolts in nominal diameters from 1/2 inch through 1-1/2 inches. (ASTM Store)
How to Read the Head of a Structural Bolt
Pick up a structural bolt and look directly at the top of the head.
You may see several letters, numbers, symbols, or lines.
Those markings are identification—not decoration.
The two things you especially want to identify are:
Grade identification
and
Manufacturer identification
For example, a bolt head may contain:
A325
plus a separate manufacturer’s identifying symbol.
Or:
A490
plus the manufacturer’s mark.
The exact layout can differ by manufacturer, so don’t memorize the physical position of every mark.
Instead, learn what the markings mean.
What Does “A325” on the Bolt Head Mean?
If the bolt is properly marked A325, that identifies it as an ASTM F3125 Grade A325 structural bolt.
This is one of the easiest structural bolt markings to recognize.
You may also see additional markings identifying the manufacturer and, depending on the bolt, other characteristics.
A325 bolts are widely used for structural steel connections such as beams, columns, braces, frames, platforms, industrial structures, and similar bolted assemblies.
On industrial projects, that can include everything from building steel to refinery structures and pipe racks.
Ironworkers are only one part of those projects. Næxon’s Refinery Turnaround Dictionary provides a useful reference for workers learning the terminology used across shutdowns, maintenance projects, and industrial facilities.
What Does “A490” on the Bolt Head Mean?
An A490 head marking identifies the higher-strength ASTM F3125 Grade A490 structural bolt.
Its minimum tensile strength is 150 ksi compared with 120 ksi for A325.
That is a substantial difference.
But remember:
The head marking identifies the bolt. It does not tell you by itself whether that bolt belongs in the connection in front of you.
The erection drawing, connection detail, bolt list, specification, or other approved project documentation determines that.
Never sort structural bolts simply by appearance.
Read the markings.
Why the Manufacturer’s Mark Matters
Along with the grade identification, structural bolts carry a manufacturer’s identification.
That mark provides traceability.
On a major structural project, fasteners are controlled materials. Inspectors and quality personnel may need to connect installed fasteners with certifications, manufacturing lots, testing documentation, or material records.
That becomes especially important when questions arise about a particular batch.
A bolt isn’t merely:
“a 7/8 bolt.”
It has a diameter, grade, type, manufacturer, finish, length, thread configuration, and associated installation requirements.
That traceability is part of why proper bolt identification matters.
Type 1 vs. Type 3 Structural Bolts
Another designation an ironworker may encounter is Type 1 or Type 3.
This is separate from the A325/A490 strength grade.
Under ASTM F3125, the type identifies material chemistry characteristics.
Type 1
Type 1 is the conventional structural bolt material category.
For A325, Type 1 may be produced from specified carbon steel, carbon-boron steel, alloy steel, or alloy steel with boron addition.
A490 Type 1 uses alloy-steel chemistry meeting the applicable requirements.
Type 3
Type 3 indicates weathering steel.
It is intended to provide atmospheric corrosion resistance when used as part of an appropriate weathering-steel system.
ASTM F3125 provides both Type 1 and Type 3 options within the applicable structural bolt grades. (ASTM Store)
This leads to an important lesson:
Grade and type are not the same thing.
A325 vs. A490 primarily tells you the strength grade.
Type 1 vs. Type 3 tells you about the material category.
How Can You Recognize a Type 3 Bolt?
Type 3 bolts have distinguishing grade identification requirements.
Depending on the applicable marking convention, weathering-type structural fasteners may include an underline or other required identification associated with the grade marking.
This is exactly why workers shouldn’t try to identify specialty fasteners based on color alone.
Weather, storage, dirt, oil, rust, coatings, and handling can change how a bolt looks.
Read the actual head marking.
If there is any uncertainty, stop and verify the bolt against the project documentation and material identification rather than guessing.
What Is an A325T Bolt?
This is another marking worth knowing.
You may encounter a bolt marked:
A325T
The T has a specific meaning.
An ASTM F3125 Grade A325T invokes the supplementary provision allowing the bolt to be fully threaded within the dimensional limitations established by the specification.
AISC notes that this provision applies to Grade A325 bolts with lengths no greater than four times the bolt diameter. (AISC)
So:
A325 = standard Grade A325 configuration
A325T = fully threaded Grade A325 under the applicable supplementary requirement
This matters structurally because a fully threaded bolt means the threads cannot be excluded from the shear plane.
That can affect the connection’s design strength.
The “T” therefore isn’t some random manufacturing mark.
It tells the engineer, inspector, fabricator, and erector something important about the bolt geometry.
What Does the “S” Mean?
You may also encounter structural bolts carrying an S with the grade designation.
The S indicates that certain dimensions differ from the standard configuration under the applicable supplementary requirement.
For example:
A325S
or
A490S
AISC specifically warns that these dimensional variations need to be considered because assumptions involving shank length, thread length, and the location of the faying surface may change. (AISC)
Again, small letters on a bolt head can carry important information.
What About Twist-Off Bolts?
Not every structural bolting assembly uses a conventional heavy-hex bolt installed with ordinary wrenches.
Ironworkers commonly encounter tension-control bolts, often called:
TC bolts
or
twist-off bolts.
Under ASTM F3125, the traditional matched twist-off structural bolt grades include:
F1852 — 120 ksi class
F2280 — 150 ksi class
These correspond broadly to the strength levels associated with A325 and A490 heavy-hex structural bolts. ASTM F3125 covers these twist-off assemblies as part of the consolidated structural fastener standard. (ASTM Store)
A TC bolt has the recognizable spline extending beyond the threaded end.
During installation with the proper tension-control installation tool, the system reacts through the nut and spline, and the spline twists off at the designed point as part of the installation process.
But don’t confuse:
Spline broke off = automatic proof that every requirement was satisfied.
Proper storage, lubrication condition, assembly condition, pre-installation verification, installation procedures, and project requirements still matter.
Structural Bolts Are Assemblies, Not Just Bolts
One of the most important concepts for new ironworkers is that the bolt isn’t working alone.
A structural bolted connection may involve a matched system of:
Bolt + nut + washer(s)
The required components depend on the bolt grade, type, finish, installation method, and project specification.
For example, common F3125 matching-component combinations include ASTM A563 structural nuts and ASTM F436 hardened washers. Specific nut grades vary according to bolt grade, Type 1 versus Type 3, and finish. (Portland Bolt)
That means you should never think:
“The bolt fits the hole, so this nut is probably fine.”
Structural fasteners aren’t miscellaneous hardware.
Use the specified compatible assembly.
Why Hardened Washers Matter
A washer underneath a structural bolt or nut may look like one of the simplest pieces in the connection.
It isn’t necessarily interchangeable with a hardware-store washer.
ASTM F436 hardened structural washers are designed for use with high-strength structural bolting systems.
Their use and location depend on factors including:
- Bolt grade
- Installation method
- Hole type
- Connection configuration
- Surface condition
- Project specification
Don’t automatically add, remove, stack, or substitute washers because “that’s how we usually do it.”
Follow the approved bolting requirements.
The same principle applies throughout industrial work: small components often have a much bigger engineering purpose than their appearance suggests. Næxon explores that broader skilled-trade mindset in Built by the Trades: Why the Best Jobsite Gear Often Starts With a Worker Saying, “I Can Make That Better”.
Can A325 Bolts Be Galvanized?
A325 bolting assemblies can be supplied with permitted protective coatings when the coating and complete assembly meet the applicable requirements.
Hot-dip galvanizing is commonly encountered with A325 structural fasteners.
But coating isn’t simply:
Take any bolt and dip it in zinc.
The fastener, nut, coating, lubrication, rotational behavior, and assembly requirements have to work together.
This becomes especially important when pretensioning structural bolts.
What About Galvanized A490 Bolts?
This is where workers need to be particularly careful.
Traditional hot-dip or mechanical galvanizing of high-strength A490/Group 150 fasteners is restricted because of concerns associated with hydrogen embrittlement and the high strength of the material.
Current F3125 requirements provide specific qualified coating provisions rather than allowing workers to assume that any conventional galvanizing process acceptable for A325 is also acceptable for A490. (Portland Bolt)
So if you see something that appears to be a galvanized A490 assembly, don’t immediately assume:
“That’s impossible.”
And don’t assume:
“It’s fine.”
Verify the actual fastener specification, coating system, certification, and project requirements.
The rules are more specific than the old jobsite shortcut of simply saying A490 bolts can never have any coating.
Snug-Tight Does Not Mean Pretensioned
Bolt identification is only the beginning.
After you’ve verified the correct fastener, you still need to know how the connection is supposed to be installed.
Structural bolted joints may have different installation requirements.
One connection may only require the specified snug-tight condition.
Another may require pretensioning.
Another may be designed as a slip-critical joint.
Those terms are not interchangeable.
And the correct bolt does not make an incorrectly installed connection acceptable.
This is one reason structural bolting deserves to be treated as its own technical skill rather than simply “putting bolts in holes.”
Never Judge Structural Bolt Tension by Feel Alone
Experienced ironworkers develop a strong feel for tools.
That’s valuable.
But structural bolt pretension shouldn’t simply be:
“That feels tight enough.”
Approved methods may include techniques such as calibrated-wrench installation, turn-of-nut, tension-control bolt installation, direct-tension indicators, or other procedures permitted by the governing specification.
The required procedure depends on the connection and project requirements.
A large impact wrench can make a bolt feel extremely tight without proving that the required installation procedure was followed.
The Rusty Bolt Myth
Another common misconception is:
“If the bolt is rusty, it’s bad.”
Surface condition needs more context.
Some structural fasteners are intentionally supplied without a shiny protective coating.
Weathering-steel Type 3 fasteners are an obvious example where the material itself is selected for atmospheric corrosion behavior.
On the other hand, severe corrosion, pitting, damaged threads, contamination, improper storage, or deterioration can absolutely make a fastener unsuitable.
The lesson is:
Appearance alone doesn’t identify acceptability.
Verify the specification and condition.
Don’t Mix Bolt Assemblies in the Bucket
Imagine several crews are erecting steel and the job uses multiple bolt grades or assemblies.
Someone dumps leftover bolts, nuts, and washers into the same bucket.
Now the next ironworker has a pile containing different components that look nearly identical.
That’s exactly the situation head markings and material-control procedures are intended to prevent.
Before installation:
Read the bolt head.
Verify the nut.
Verify the washer when required.
Verify the diameter and length.
Confirm the specified assembly.
It takes seconds compared with removing incorrectly installed structural bolts later.
Common Structural Bolt Identification Mistakes
Mistake 1: Identifying the bolt by color
Black does not automatically mean A325.
Silver does not automatically tell you everything about the coating or grade.
Read the markings.
Mistake 2: Assuming every heavy-hex bolt is structural
Head shape alone isn’t enough.
Mistake 3: Assuming A490 can replace A325 because it is stronger
Substitution requires engineering/project approval.
Mistake 4: Ignoring the nut
The nut is part of the structural bolting assembly.
Mistake 5: Mixing Type 1 and Type 3 components
Weathering-steel assemblies require the appropriate compatible components.
Mistake 6: Ignoring extra letters
A325T and A325S aren’t meaningless variations.
Those letters identify important differences.
Mistake 7: Assuming bolt tension from impact-wrench sound
Installation must follow the approved method—not the sound of the gun.
A Simple Field Identification Routine
When you grab a structural bolt, develop the habit of checking it before installing it.
1. Read the head marking.
Is it A325? A490? Something else?
2. Check the manufacturer’s identification.
Make sure the fastener has the required identification.
3. Check for additional markings.
Look for Type 3 identification, T, S, or other applicable markings.
4. Verify diameter and length.
The correct grade in the wrong size is still the wrong bolt.
5. Verify the nut and washer requirements.
Don’t build an assembly from random hardware.
6. Check the condition.
Look for damaged threads, severe corrosion, contamination, coating damage, or anything else that could affect installation.
7. Check the erection drawings and project bolting requirements.
The drawing tells you what belongs in the connection.
A325 vs. A490: The Field Memory Trick
If you only remember the basics, remember this:
A325
120 ksi minimum tensile strength
Common high-strength structural bolt.
A490
150 ksi minimum tensile strength
Higher-strength structural bolt.
Type 1
Conventional structural bolt material category.
Type 3
Weathering-steel fastener.
T
Fully threaded A325 configuration under the applicable supplementary requirement.
S
Nonstandard dimensional variation permitted under the applicable supplementary requirement.
And above everything else:
Read the head before you run the bolt.
Why This Matters Beyond Ironworking
Structural steel exists throughout heavy industry.
Refineries have pipe racks, platforms, equipment structures, towers, access steel, buildings, and support frames.
Power plants contain enormous amounts of structural steel.
Data centers, chemical plants, manufacturing facilities, compressor stations, mines, steel mills, and industrial construction projects all rely on structural connections.
That means structural bolting knowledge isn’t useful only to ironworkers.
Millwrights, boilermakers, riggers, inspectors, supervisors, welders, engineers, and maintenance crews regularly encounter structural fasteners.
If you’re building a broader understanding of industrial work, the Næxon Resources & Learning Center collects trade knowledge covering industrial construction, maintenance, fabrication, safety, tools, and field practices.
Industrial workers can also explore Næxon’s guide to companies hiring for refinery, turnaround, and industrial jobs across the U.S. to see how ironworkers fit alongside the many other crafts required on major industrial projects.
The Bolt Head Is Talking to You
A structural bolt is a small component holding together something that may weigh thousands—or millions—of pounds.
The markings on its head tell you what you’re holding.
A325 and A490 aren’t just numbers.
They identify different strength grades.
Type markings tell you more about the material.
Additional letters can tell you about thread or dimensional configuration.
Manufacturer marks provide traceability.
Once you understand those markings, a bucket of structural bolts stops looking like a pile of identical hardware.
You start seeing exactly what the engineer, fabricator, inspector, and experienced ironworker sees:
A controlled structural fastener with a specific job to do.
And before that bolt ever gets hit with an impact, the first step is always the simplest:
Make sure it’s the right bolt.
