Hex bolt grades compared: Class 8.8 vs 10.9 vs 12.9 vs ASTM A307 / A325 / A490
Choosing a hex bolt grade is a procurement decision, not a brand choice: the grade sets the minimum strength, the allowable coating and the inspection basis of the joint. This table compares the six grades we publish individual specification pages for — three ISO property classes and three inch-series ASTM grades — using only fields that mean the same thing and are measured the same way across the columns. Where a figure is not established by the collected evidence, the cell says so and asks you to confirm it at RFQ; nothing is filled in with an assumption, and no cell implies the grade cannot be supplied.
Comparison table
All values are minimums per the governing standard named in each column. Metric properties are defined by ISO 898-1; inch-series properties by the ASTM specification — the two families are tabulated side by side for procurement orientation, and a substitution between them should always be confirmed with the engineer of record (see the FAQ below).
| Field | ISO Class 8.8 | ISO Class 10.9 | ISO Class 12.9 | ASTM A307 Gr. A | F3125 Gr. A325 Type 1 | F3125 Gr. A490 |
|---|---|---|---|---|---|---|
| Min tensile strength Minimum per the governing standard; ISO in MPa, ASTM tabulated in psi with the MPa equivalent. Size-dependent values shown per size group. ASTM F3125/F3125M states its inch-pound and SI values are not exact equivalents — the MPa figures here are arithmetic reading references only; ordering, acceptance and design must use the original table values in the unit system your drawing specifies. | 800 MPa (≤ M16); 830 MPa (> M16) | 1,040 MPa | 1,220 MPa | 60,000 psi (≈ 414 MPa); Grade B capped at 100,000 psi max | 120,000 psi (≈ 827 MPa, ≤ 1 in); 105,000 psi (≈ 724 MPa, > 1 in) | 150,000 psi (≈ 1,034 MPa) |
| Proof load / min yield Same measurement basis per column: metric columns carry ISO 898-1 proof load; the structural column carries ASTM proof load. A307 specifies no minimum yield — recorded as unverified, not as zero. | Proof load 580 MPa (yield 640 MPa ≤ M16) | Proof load 830 MPa (yield 940 MPa) | Proof load 970 MPa (yield 1,100 MPa) | Unverified — A307 does not tabulate a minimum yield; confirm at RFQ Unverified — confirm at RFQ | Proof load 85,000 psi (≤ 1 in); 74,000 psi (> 1 in) | Unverified — F3125 tabulates A490 tensile only in the collected evidence; confirm at RFQ Unverified — confirm at RFQ |
| Hardness Hardness scales are NOT directly comparable across rows — Rockwell C for the quenched grades, Brinell for A307. Scale differences are marked, not converted. | 22–32 HRC | 32–39 HRC | 39–44 HRC | B69–B100 (Brinell — different scale from HRC, not directly comparable) | C24–35 (≤ 1 in); C19–31 (> 1 in) | Unverified — confirm at quotation Unverified — confirm at RFQ |
| Material & heat treatment Same meaning across columns (base steel and processing); the metric columns state the ISO 898-1 class definition, the inch columns the ASTM call-out. | Medium carbon steel, quenched and tempered | Alloy steel, quenched and tempered | High alloy steel, quenched and tempered | Low or medium carbon steel, no quench and temper | Medium carbon steel, quenched and tempered (≥ 800 °F temper) | Alloy steel, quenched and tempered |
| Hot-dip galvanizing Same meaning: whether the grade accepts hot-dip galvanizing. Where the standard constrains it, the constraint is the value — it is not a supplier limitation. | Zinc plate and hot-dip galvanized are quoted finishes for this grade — confirm the coating system at RFQ Conditional — confirm at RFQ | Accepted (post-plate bake for hardened parts) | Not the recommended path — temper and embrittlement risk; zinc flake preferred. Confirm coating with the drawing Conditional — confirm at RFQ | Accepted | Accepted — Type 1 is the A325 type that can be galvanized | Not permitted by the standard (hydrogen embrittlement risk); protect by paint or Type 3 weathering steel |
| Typical applications Same meaning: the service the grade is specified for. Applications are evidence-derived typicals, not a guarantee of suitability for a specific joint. | Machine construction, structural connections, automotive, general engineering | High-strength structural connections, heavy-duty machinery, chassis hardware | Precision machinery, extremely high-stress assemblies, tooling | Light framing, equipment anchoring, pipe flanges (Grade B), general construction | Structural steel joints in bridges, highway and heavy construction | Very heavy and critical structural connections where A325 strength is insufficient |
- Specified
- Differs — read the note
- Unverified — confirm at RFQ
- Conditional — confirm at RFQ
How to choose
Within the metric classes (8.8 / 10.9 / 12.9)
The class number is the minimum tensile in 100 MPa units, so the choice is driven by the joint calculation: 8.8 for general machine and equipment work, 10.9 where the load or weight budget needs roughly 30% more tensile, 12.9 where maximum clamp load must fit a compact envelope. Strength above what the joint needs is not free — the higher classes are less ductile and progressively more sensitive to hydrogen embrittlement, which narrows the coating options (12.9 is normally zinc-flake coated rather than hot-dip galvanized).
Within the inch-series grades (A307 / A325 / A490)
A307 is the economical low-strength grade for static, non-structural work; the moment a connection is engineered to a structural code, the specification moves to F3125 Grade A325, and to A490 where A325's clamp load is insufficient. Coating strategy is part of the same decision: A325 Type 1 can be hot-dip galvanized, Type 3 is weathering steel supplied bare, and A490 cannot be galvanized at all — it is protected by paint systems or specified as Type 3.
Across the two families
A metric class and an inch-series grade are not interchangeable just because two numbers look close: the test methods, proof-load definitions and installation practices differ. Order the designation the drawing names. If your supply chain is metric-tooling based and the drawing allows equivalents, confirm the agreed designation with the engineer of record before the RFQ — then quote it explicitly.
Frequently asked questions
Can I substitute a Class 10.9 bolt where the drawing calls for ASTM A490 (or the other way)?
Not on your own initiative. Their minimum tensile is numerically close (1,040 MPa vs 150,000 psi ≈ 1,034 MPa), but the standards differ in test method, proof-load definition, hardness window and mandated installation practice — a metric class is not an automatic equivalent for an inch-series structural grade. Never substitute on a drawing that names a specific standard; if a cross-grade proposal makes sense for your joint, confirm it with the engineer of record and state the agreed designation on the RFQ.
Why do some cells say "Unverified — confirm at RFQ" instead of a figure?
This table only states figures the collected evidence supports. Where a standard does not tabulate a property (A307 has no minimum yield) or the collected evidence does not cover it, the cell is marked unverified rather than filled with an assumed number. Request the current mill test report or grade chart with your RFQ and the figure is confirmed against the shipped lot.
How do I choose between 8.8, 10.9 and 12.9 for the same joint?
By the joint calculation, not by habit. 8.8 is the forgiving default for general machine and equipment work; 10.9 buys roughly 30% more tensile for heavily loaded or weight-sensitive joints at the cost of higher embrittlement sensitivity; 12.9 maximizes clamp load in a compact envelope and demands strict coating practice (zinc flake rather than hot-dip galvanizing). If the load case is comfortably within 8.8, the lower grade is normally the more robust choice.
Is DIN 931/DIN 933 still the right designation to order metric hex bolts?
DIN 931 (partially threaded) and DIN 933 (fully threaded) hex bolts have been superseded by ISO 4014 and ISO 4017 respectively; drawings still name both. The thread choice inside the subtype is the real decision: fully threaded (DIN 933 / ISO 4017) gives nut-position flexibility across varying grip lengths, while partially threaded (DIN 931 / ISO 4014) keeps an unthreaded shank out of the shear plane where the joint carries transverse load. State either designation on the RFQ — quote against the current ISO standard.
Individual grade specification pages
Each grade has its own page — full mechanicals, ordering details and RFQ guidance.
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What to include in your inquiry
- ✓ Standard and property class (e.g. DIN 912, ISO 4762, class 8.8) — or a drawing for custom parts.
- ✓ Sizes, lengths and thread type (metric or inch).
- ✓ Quantity per size and your target delivery date.
- ✓ Destination country, plus any coating or finish requirement.
- ✓ Any documents you need with the shipment (e.g. mill test certificate) so the quote includes them.
- ✓ Your inquiry goes straight to the factory team by email and gets a reply within 24 hours.
- ✓ Factory-direct pricing
- ✓ Standard and custom specifications
- ✓ Factory-direct export to buyers in Europe, Asia, the Americas and the Middle East
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