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Hex bolt
hex bolt is a headed fastener with a hexagonal driving surface and a threaded shank, usually with an unthreaded grip under the head. The hex profile takes high torque from a wrench or socket, which is why hex bolts dominate structural steel and heavy equipment. For the wider head geometry picture, see our guide to .
Five features matter: the head (across-flats sets wrench size); the bearing surface (plain, or a machined washer face on a cap screw); the threaded shank; the grip length (the smooth section in the shear plane); and the tip (flat on bolts, chamfered on cap screws). The external hex is one of several and resists cam out under heavy torque.
Bolts earn their keep by stretching slightly under tension to create clamp force. As long as clamp stays above the separating load, the joint acts like one solid piece. When clamp drops, the joint slips. All of that rolls up into one number: preload.
Start with the carriage bolt: a rounded head with a square neck that bites wood. Next, the lag bolt is coarse threaded and blunt tipped, driven by its own threads into wood or masonry with no nut. By contrast, a hex bolt clamps with a nut or taps a prepared hole and is built to a published standard; swapping in a carriage or lag usually voids the design.
Hex bolts split into six main types: fully and partially threaded bolts, heavy hex bolts, flange hex bolts, structural hex bolts (F3125/A325/A490), and prevailing-torque variants. Hex cap screws are a tighter-tolerance cousin covered separately.
Go fully threaded and the bolt runs threads to the head for max engagement in tapped holes and thin stacks. A partially threaded bolt keeps an unthreaded grip, the workhorse of shear joints because thread roots are notches and the weakest section. Spec rule: where the joint sees shear, specify partial thread and size the grip to cover the shear plane.
Heavy hex is the beefed-up cousin: its head runs wider and taller than a standard hex at the same diameter (at 3/4 inch, 1-1/4 inch versus 1-1/8 inch). The extra metal spreads load and resists wrench slip; heavy hex bolts are the basis of structural steel work to ASTM F3125 (the home of A325 and A490).
The hex flange bolt builds the washer in: an integrated bearing surface fights vibration loosening, ideal for pump and compressor skids where fewer parts help. Flange bolts follow their own standards (such as DIN 6921).
Structural hex bolts are heavy hex fasteners to F3125, with A325 for general structural steel and A490 for the highest. They install with a hardened washer under head and nut; the washer and nut are part of the system.
Prevailing-torque types fight back on their own: they resist rotation without a separate lock nut, usually through a distorted thread or a bonded patch. They hold in high vibration service where a standard nut would walk off. Use them where vibration is real and document the method, as the feature adds torque and can mask proper preload.
The hex cap screw wears the same six sides but is machined to tighter tolerances under ASME B18.2.1, usually carries a washer face, and is meant for a tapped hole. The two are not interchangeable in precision or code driven joints. We cover the DIN 933/931 versus ASME B18.2.1 detail and socket head options in the guide. For the contrast with recessed drives, see
Metric hex bolt dimensions are called out as diameter, pitch, and length, for example M16 x 2 x 80. The table below is a paste ready reference for the common range, built from ISO 4014 (partial) and ISO 4017 (full).
Hex bolt dimensions follow two systems. Metric sizes use diameter, pitch, and length (for example M16 x 2 x 80); imperial sizes use fractional diameter, TPI, and length in inches. The tables below are paste-ready references built from ISO 4014/4017 and ASME B18.2.1.
| Nominal dia (M) | Coarse pitch (mm) | Across flats (mm) | Head height (mm) | Thread length b, partial (mm) |
|---|---|---|---|---|
| M6 | 1.0 | 10 | 4.0 | 18 |
| M8 | 1.25 | 13 | 5.3 | 22 |
| M10 | 1.5 | 16 | 6.4 | 26 |
| M12 | 1.75 | 18 | 7.5 | 30 |
| M16 | 2.0 | 24 | 10.0 | 38 |
| M20 | 2.5 | 30 | 12.5 | 46 |
| M24 | 3.0 | 36 | 15.0 | 54 |
| M30 | 3.5 | 46 | 18.7 | 66 |
| M36 | 4.0 | 55 | 22.5 | 78 |
Values are approximate per ISO 4014/4017 and rounded for shop use. Confirm against the standard for certified work.
Imperial bolts use fractional diameter, threads per inch (TPI), and length in inches. Coarse is UNC; fine is UNF.
| Nominal dia | UNC (TPI) | Across flats | Head height |
|---|---|---|---|
| 1/4″ | 20 | 7/16″ | 11/64″ |
| 5/16″ | 18 | 1/2″ | 7/32″ |
| 3/8″ | 16 | 9/16″ | 1/4″ |
| 1/2″ | 13 | 3/4″ | 5/16″ |
| 5/8″ | 11 | 15/16″ | 25/64″ |
| 3/4″ | 10 | 1-1/8″ | 15/32″ |
| 7/8″ | 9 | 1-5/16″ | 35/64″ |
| 1″ | 8 | 1-1/2″ | 39/64″ |
| 1-1/4″ | 7 | 1-7/8″ | 25/32″ |
| 1-1/2″ | 6 | 2-1/4″ | 15/16″ |
Dimensions per ASME B18.2.1. The inch series spans roughly 1/4″–1-1/2″ in the common structural and machinery range, with larger sizes available.
Coarse (UNC, or coarse metric pitch) is the field default: it assembles faster, shrugs off dirt, and resists cross threading. Fine (UNF, or fine metric pitch) gives roughly 5 to 10 percent more preload at the same torque, holds better under vibration, and suits thin sections and automotive work. Do not mix the two in one joint.
Strength lives in the numbers. For Class 8.8, the first digit times 100 gives nominal tensile strength in MPa (about 800). The second digit times 10 gives the yield to tensile ratio (8.8 yields at roughly 640 MPa).
Hex bolt grades encode strength. Metric property classes (ISO 898-1) read as two numbers: the first x100 gives tensile strength in MPa, the second x10 gives the yield ratio. The table below maps Class 4.6 through 12.9 to typical use.
| Property class | Tensile strength (MPa) | Yield strength (MPa) | Typical use |
|---|---|---|---|
| 4.6 | 400 | 240 | Mild steel, non structural, light duty |
| 8.8 | 800 | 640 | General machinery, structural, flanges |
| 10.9 | 1000 | 900 | High load, automotive, slip critical |
| 12.9 | 1200 | 1080 | Maximum strength, hydraulic, tooling |
Imperial grades mark strength with radial lines: Grade 2 (no markings, about 74,000 psi, hardware); Grade 5 (three radial lines, about 120,000 psi, the workhorse); Grade 8 (six radial lines, about 150,000 psi, suspension and structural).
ASTM A307 is the low carbon commercial bolt, about 60,000 psi, for general purpose and non critical members. A325 and A490 are the high strength structural grades, now folded into ASTM F3125, which means the whole system: heavy hex head, nut, washer, and a documented tension method.
Paste this into your inspection sheet: Metric digits “8.8”, “10.9”, or “12.9”; SAE no lines for Grade 2, three radial lines for Grade 5, six for Grade 8; ASTM “A325” or “A490” plus a manufacturer mark, while A307 is often marked “A307” or with a single dot.
Reach for 8.8 for general machinery, flanges, and motor mounts where loads are moderate. Step up to 10.9 for slip critical, high load, or weight limited joints such as automotive powertrain. Do not default to the highest number; a 12.9 bolt is brittle under shock.
ISO 4014 covers the partially threaded hex bolt; ISO 4017 covers the fully threaded version. ISO 4014 and 4017 are the global default for metric hex bolts and your dimensional inspection documents.
DIN 933 is the German full thread standard and DIN 931 the partial thread standard. They precede the ISO pair: DIN 933 aligns with ISO 4017, and DIN 931 aligns with ISO 4014. Many drawings still call out the DIN numbers, so knowing the mapping prevents a mismatched order.
ASME B18.2.1 governs the inch series hex cap screw and hex bolt. ASME B18.2.6 governs the heavy hex structural bolt, the dimensional home of A325 and A490. If you build to US or Canadian codes, B18.2.6 is the one your inspector will cite.
EN 24014 and EN 24017 are the European harmonized equivalents: EN 24014 matches ISO 4014 (partial) and EN 24017 matches ISO 4017 (full). For exporters shipping into the EU, citing the EN number on the certificate satisfies the harmonized requirement that the bare ISO number alone may not. This EN coverage is where GUXIANG gives you an edge most competitor guides never mention.
| Thread coverage | ISO | DIN | ASME | EN |
|---|---|---|---|---|
| Full thread | ISO 4017 | DIN 933 | ASME B18.2.1 (inch) | EN 24017 |
| Partial thread | ISO 4014 | DIN 931 | ASME B18.2.1 (inch) | EN 24014 |
| Heavy hex structural | ISO 4775 | DIN 6914 | ASME B18.2.6 (F3125) | EN 14399 (sets) |
Carbon steel is the default: cheap, strong, easy to heat treat, but it rusts in wet or outdoor service and needs a coating. Our guide covers options; carbon steel plus the right finish covers most bolted joints.
Stainless hex bolts are called out as A2-70 (304) or A4-80 (316), about 700 MPa and 800 MPa tensile. It is not as strong as alloy steel but removes the coating question in corrosive service. The trap is galling: stainless threads seize under dry high torque, so lubrication matters more here than anywhere. For the 304 versus 316 call, our guide lays out the chloride threshold where 316 earns its premium.
Alloy steel sits behind Class 10.9 and 12.9. It is quenched and tempered for high strength, the choice for engine, suspension, and hydraulic joints. The trade is brittleness under shock and a hard no on some coatings. Our note helps when weight, not strength, drives the design, though aluminum hex bolts are rare and light duty only.
Brass and silicon bronze show up in electrical and marine work needing corrosion resistance without magnetic or galvanic conflict. Match the metal to the mating parts to avoid galvanic corrosion, and when in doubt read our
Zinc plating lays down 5 to 12 microns of zinc for indoor and mild atmospheric service (roughly 72 to 200 hours salt spray), the default for indoor machinery and the cheapest clean defense.
HDG drops the bolt in molten zinc for a 45–85 micron coating that survives outdoor and structural exposure for decades. The thick coating changes the thread form, so a galvanized bolt needs a galvanized nut tapped to the oversized tolerance. Do not hot dip galvanize Class 12.9: the acid pickling step can introduce hydrogen, and a 1200 MPa bolt is the one that fails by hydrogen embrittlement. Keep HDG to carbon steel and alloy up to about Class 10.9, with a controlled bake out.
Zinc flake coatings such as Dacromet and Geomet give 1000 plus hours salt spray with no hydrogen embrittlement risk, because there is no acid pickling. That is why they are the preferred finish for Class 10.9 and 12.9 bolts in automotive, wind, and marine service, though they cost more than zinc plate.
Black oxide and phosphate are thin, mostly cosmetic finishes with low corrosion resistance. They suit an engine bay or display unit but need oil, offering little standalone protection; do not specify them for anything that sees weather.
| Environment | Best finish | Why |
|---|---|---|
| Indoor, dry | Plain or black oxide | No corrosion driver; cost first |
| Mild outdoor | Zinc plate | Cheap, adequate salt spray life |
| Harsh outdoor, structural | HDG (to Class 10.9 max) | Thick, long life, field proven |
| Automotive, wind, marine | Zinc flake (Dacromet/Geomet) | No H2 risk, 1000+ hours |
| Chloride, seawater | A4-80 stainless or zinc flake | Resists pitting and crevice attack |
Where a seated joint meets a countersunk feature, our
| Application | Load | Environment | Type | Grade | Material | Finish |
|---|---|---|---|---|---|---|
| General machinery | Moderate | Indoor | Partial hex | 8.8 / Grade 5 | Carbon steel | Zinc plate |
| Structural steel frame | High, slip critical | Outdoor | Heavy hex, F3125 | A325 / A490 | Carbon steel | HDG |
| Pump or compressor skid | Moderate, vibration | Indoor or washdown | Flange bolt | 10.9 / Grade 8 | Carbon or alloy | Zinc flake |
| Marine or chemical | Moderate | Chloride | Partial hex | A4-80 | 316 stainless | Plain (stainless) |
| Automotive powertrain | High | Underbody | Partial hex | 10.9 | Alloy steel | Zinc flake |
| Precision tapped hole | Moderate | Indoor | Hex cap screw | 8.8 / Grade 5 | Carbon or alloy | Zinc plate |
Outdoor steel frame. Heavy hex A325, HDG, carbon steel, with heavy hex nut and hardened washer. The heavy head spreads load, HDG fights weather, and F3125 gives the inspector a tension path.
Pump housing. Flange bolt, Class 10.9, zinc flake. The flange replaces a loose washer, 10.9 holds preload, and zinc flake avoids hydrogen risk.
Marine handrail. Partial hex, A4-80 stainless, plain. No coating to fail, and 316 resists the chloride that would eat carbon steel in a season.
Engineers spec the bolt; buyers make sure the box matches. Before you release payment:
Mill Test Report (MTR). Demand a report showing actual chemistry and mechanical results, not just the grade claim.
Dimensional inspection. Verify across flats, head height, thread pitch, and length against the standard on a sampled basis.
Certifications. Confirm ISO 9001 for the plant, and CE or the relevant harmonized standard (such as EN 24014/24017) for EU shipment. Check RoHS where restricted substances apply.
Traceability. The lot number on the package should tie back to the MTR and the heat.
One buyer I work with skipped the mill test report to save two days on a container of Class 10.9 bolts. Third-party inspection found them 12 percent under the declared proof load. The lot was rejected, and the line sat idle for a week. The two days saved turned into eleven days lost. Paperwork is the part of the product you cannot see until it fails.
Hex bolt torque is the lever you pull to reach preload; get the relationship wrong and a perfectly graded bolt still fails.
In short: T = K × d × F, where T is torque, K the nut factor, d the nominal diameter, and F target preload. K swallows every real world variable: friction under the head, in the threads, and surface condition. Use K = 0.20 dry, K = 0.15 lubricated, and K near 0.13 for anti seize or wax; the same bolt can need 35 percent less torque once lubricated.
Hex bolt torque depends on grade and lubrication. The same bolt can need up to 35% less torque once lubricated. The table below lists dry and lubricated Nm for Grade 8.8 and 10.9 from M6 to M24.
| Size | Grade 8.8 dry (Nm) | Grade 8.8 lube (Nm) | Grade 10.9 dry (Nm) | Grade 10.9 lube (Nm) |
|---|---|---|---|---|
| M6 | 10 | 7 | 14 | 10 |
| M8 | 25 | 18 | 35 | 25 |
| M10 | 50 | 35 | 70 | 50 |
| M12 | 85 | 60 | 120 | 85 |
| M16 | 210 | 150 | 300 | 210 |
| M20 | 410 | 290 | 580 | 410 |
| M24 | 700 | 500 | 1000 | 700 |
Treat the table as a starting point, not the final word; a lubricated stainless joint can need half the dry torque before galling begins.
Dry stainless threads gall: the metal welds to itself under turning load and the head spins while the shank stays put, freezing the joint. A dab of anti seize on stainless is the difference between a clean install and a drilled out bolt. Lubrication also makes your torque reading mean something; without a known K, torque is just a guess at preload.
Aim for 70–90% of proof load in a properly prepared joint. Below 70 percent clamp may not beat the separating load; above 90 percent you crowd yield and small errors snap the bolt. For structural F3125 work, follow the turn of nut or direct tension method in the project spec rather than a torque wrench alone.
Most joint failures trace back to a short list of spec errors. Most surface in the field, not on the drawing.
Mixing coarse and fine, or metric and imperial. The diameters look close and the threads strip. Confirm the system before you order.
Threads in the shear plane. A fully threaded bolt in a shear joint loads through thread roots. Specify partial thread and size the grip to cover the shear plane.
Hot dip galvanizing Class 12.9. The pickling step risks hydrogen embrittlement in the one grade that cannot tolerate it. Cap HDG at about Class 10.9 with bake out.
Over torquing dry stainless. Dry stainless galls and freezes. Lubricate it and torque to the lubricated value.
Substituting without checking yield and standard. A bolt that looks right can be a different grade or standard. Verify the head mark and the certificate.
Skipping the paperwork.
and cover the fasteners that are not hex bolts at all.
A: In practice: reach for a hex cap screw when you have a tapped hole and no access to the back; reach for a hex bolt when a nut does the clamping.
A: There is no single standard grade. Class 8.8 is the common high-tensile metric entry point; Class 4.6 is mild steel for non-structural use. In imperial, SAE Grade 5 is the go-to and Grade 2 is low-strength hardware.
A: Measure the nominal diameter across the thread crest and the length from directly under the head to the tip; the head height never counts. For the thread, count TPI (imperial) or measure pitch in millimetres (metric).
A: Heavy hex bolts are used for structural steel, bridges, pressure vessels, and transmission towers. They have a wider and taller head than standard hex bolts and are specified to ASTM F3125 (A325 or A490).
A: Yes. A hex bolt will thread into a tapped hole, but it is designed for nut-and-washer assemblies through clearance holes. For a precision or code controlled joint, a hex cap screw with tighter tolerances and a washer face is the safer choice.
A: Those are metric property classes to ISO 898-1. The first number times 100 gives tensile strength in MPa (8.8 is about 800); the second times 10 gives the yield-to-tensile ratio (8.8 yields at roughly 640 MPa). Class 10.9 is stronger.
A: Stainless is lower in outright strength than hardened alloy steel, but not weak. A2-70 (304) reaches about 700 MPa and A4-80 (316) about 800 MPa, suiting most atmospheric and marine service. Specify a coated high-tensile bolt for both strength and corrosion.
A: Yes, but only if both are hot-dip galvanized to matching oversized thread tolerances. Galvanizing thickens the threads, so it needs a galvanized nut tapped to the special class. Pairing it with a plain nut will likely bind or strip the threads.
A: As a dry reference, an M12 Class 8.8 bolt takes roughly 75–85 Nm to reach about 75 percent of proof load at a nut factor near 0.20. Lubrication drops that figure substantially, so torque to the actual surface and verify with a calibrated wrench.
A:
When your next project needs hex bolts specified and sourced, get a quote from GUXIANG
governs hexagon head bolt dimensions, tolerances, and product grades A and B.
specifies hexagon head screw dimensions and mechanical grades for full and partial thread forms.
covers high-strength structural bolting for steel-to-steel connections.
is retained for legacy reference as the predecessor specification for structural bolts.
defines inch-series dimensions and thread requirements for hex and heavy hex fasteners.
provide German national specifications for fully and partially threaded hexagon head bolts.
are the CEN-published European versions of the ISO hexagon head bolt and screw standards.
