The wrong bolt for an application rarely fails immediately. It fails under load, under vibration, or after a season of outdoor exposure — at which point the joint is compromised, the bolt is difficult to remove, and the cost of the mistake is far higher than the cost of specifying correctly. The most common errors are not dramatic: a full-thread bolt used in a shear joint; a zinc-plated bolt in a coastal application; a coach bolt where a coach screw was needed. Each is avoidable with basic knowledge of what these fasteners are designed to do.
This guide covers hex bolt anatomy, the critical partial-thread versus full-thread distinction, the standard metric sizes and spanner sizes, finishes, and the closely related fastener types — coach bolts, flange bolts, and coach screws — that are frequently confused with hex bolts in Australian trade and industrial practice. For detailed information on bolt grades (4.6, 8.8, 10.9, 12.9) and the torque reference chart, see the AIMS Industrial Bolt Grade Chart.
What is a hex bolt?
A hex bolt is a threaded fastener with a hexagonal head and a cylindrical shank. The head has six flat faces — "hex" from the Greek for six — which accept a spanner, socket, or ring spanner for tightening and loosening. The shank is partially or fully threaded, and in normal use the bolt passes through unthreaded clearance holes in the joined material and is secured by a nut on the opposite side.
The key components of a hex bolt are:
- Head — hexagonal, provides the bearing surface and the wrench engagement. Head dimensions (across flats, across corners, head height) are standardised per ISO 4014/4017 and DIN 931/933.
- Shank — the body of the bolt. In a partial-thread bolt, the shank has an unthreaded section (the grip length) directly under the head, followed by a threaded section. In a full-thread bolt, the thread runs the entire length.
- Grip length — the unthreaded shank length. In a correctly designed bolted joint, the grip length equals the combined thickness of the joined materials, so the shear plane falls in the unthreaded shank rather than the threaded section.
- Thread — metric coarse thread is the AU standard for general fastening. Fine thread is available for vibration-critical or precision applications.
- Thread length — for partial-thread bolts, the thread occupies approximately the last 2d + 6mm of bolt length (for bolts up to 125mm). A full-thread bolt has thread to the head.
Hex bolts are the most common heavy fastener type in Australian industrial, structural, and mechanical applications. They are dimensioned under the ISO 4014 (partial thread) and ISO 4017 (full thread) international standards, also designated DIN 931 and DIN 933 respectively — both designations appear on Australian supplier packaging.
Partial thread vs full thread hex bolts
This is the most consequential distinction in hex bolt selection and the one most frequently ignored when ordering. Partial thread and full thread bolts look almost identical and are supplied in the same sizes — but their structural behaviour under shear loading is fundamentally different.
Partial thread (DIN 931 / ISO 4014)
The partial-thread hex bolt — also called a hex set bolt, hex bolt, or hex set screw (partially threaded) — has an unthreaded shank beneath the head. The thread occupies only the end portion of the bolt. When this bolt is correctly sized for the joint, the grip length equals the joint thickness, and the threaded section extends into the nut beyond the joint. The shear plane — the plane along which shear forces act — falls in the unthreaded shank.
The unthreaded shank is the full nominal diameter and has no stress concentration from thread roots. It provides the maximum cross-sectional area for resisting shear forces, and the smooth shank surface allows precise location of the joined components. Partial thread hex bolts are the correct choice for structural steel connections, machinery bases, flanges, and any bolted joint designed to carry shear or combined shear-and-tension loading.
Full thread (DIN 933 / ISO 4017)
The full-thread hex bolt — also called a hex set bolt (fully threaded) or hex set screw — has thread running the entire length from head to tip. There is no unthreaded shank. Full thread bolts are the correct choice when maximum thread engagement is the design requirement: bolting into tapped holes (no nut), through-bolting in applications where the shear plane must be avoided, or where joint movement during assembly requires full thread adjustment.
In a through-bolting application under shear load, a full-thread bolt places the thread roots — points of stress concentration — at the shear plane. Thread roots reduce the effective cross-sectional area compared to the unthreaded shank of a partial-thread bolt of the same nominal diameter. For shear-loaded joints, this is a design weakness.
Partial thread vs full thread — comparison
| Property | Partial thread (DIN 931 / ISO 4014) | Full thread (DIN 933 / ISO 4017) |
|---|---|---|
| Also called | Hex bolt, hex set bolt (p/t), set screw (p/t) | Hex set bolt (f/t), hex set screw, hex cap screw |
| Shank | Unthreaded grip length + threaded end | Thread to head — no unthreaded shank |
| Shear plane | Falls in unthreaded shank — maximum area | Falls in threaded section — reduced area, stress concentrations |
| Best for | Structural steel, machinery, flanges, shear/tension joints | Tapped holes, maximum thread engagement, clamping applications |
| Standard | ISO 4014 / DIN 931 | ISO 4017 / DIN 933 |
| Common finish | Zinc plate, HDG, plain | Zinc plate, stainless, plain |
Hex bolt sizes: dimensions and spanner chart
Metric coarse thread is the standard in Australian industrial and construction applications. The table below gives the key dimensions for metric hex bolts to ISO 4014 / ISO 4017 (DIN 931 / DIN 933) from M6 to M36 — the range covering the vast majority of AU trade and industrial fastening. These values apply to both partial-thread and full-thread hex bolts of the same nominal size.
"Across flats" (AF) is the dimension across opposite flat faces of the head — the size of spanner or socket required. "Across corners" (AC) is the maximum width across the hex head corners, relevant for clearance. "Head height" (k) is the height of the head.
| Nominal size | Coarse pitch | AF — across flats (spanner size) | AC — across corners | Head height (k) | Notes |
|---|---|---|---|---|---|
| M6 | 1.0 mm | 10 mm | 11.5 mm | 4.0 mm | Light fittings, guards, covers |
| M8 | 1.25 mm | 13 mm | 15.0 mm | 5.3 mm | Light machinery, electrical panels |
| M10 | 1.5 mm | 17 mm | 19.6 mm | 6.4 mm | General mechanical — very common |
| M12 | 1.75 mm | 19 mm | 21.9 mm | 7.5 mm | Structural, motor bases, flanges |
| M14 | 2.0 mm | 22 mm | 25.4 mm | 8.8 mm | Less common — check application |
| M16 | 2.0 mm | 24 mm | 27.7 mm | 10.0 mm | Structural steel connections |
| M20 | 2.5 mm | 30 mm | 34.6 mm | 12.5 mm | Heavy structural, base plates |
| M24 | 3.0 mm | 36 mm | 41.6 mm | 15.0 mm | Heavy structural, AS 4100 connections |
| M27 | 3.0 mm | 41 mm | 47.3 mm | 17.0 mm | Slip-critical connections, heavy equipment |
| M30 | 3.5 mm | 46 mm | 53.1 mm | 18.7 mm | Heavy industrial, pressure equipment |
| M36 | 4.0 mm | 55 mm | 63.5 mm | 22.5 mm | Very heavy structural, crane components |
The spanner (AF) size is what you order tooling against. M10 takes a 17mm spanner; M12 takes a 19mm; M16 takes a 24mm. These are the sizes to verify before starting a job on unfamiliar equipment — the wrong spanner size rounds the head corners, making removal progressively more difficult.
Fine thread variants
Fine-thread metric hex bolts are available for the same nominal diameters but with a smaller thread pitch (e.g., M12 × 1.25 vs M12 × 1.75 coarse). Fine thread develops approximately 5–10% higher preload at the same torque and offers better vibration resistance. Fine-thread bolts are used in automotive applications (suspension components, engine fasteners), precision machinery, and any application where vibration loosening of coarse thread is a concern. Do not mix coarse and fine thread in the same joint — the threads are not interchangeable.
How to measure a hex bolt
Hex bolt size is specified as nominal diameter × length (e.g., M12 × 75). Length is measured from under the head to the tip — the full shank length including the threaded portion. The head is not included in the length measurement. To identify an existing bolt: measure the shank diameter with calipers (the nominal diameter), count the thread peaks per 10mm to determine pitch, and measure from under the head to the tip for length.
Hex bolt grades
Hex bolt grade determines the strength — specifically the tensile and yield strength of the fastener material. The metric property class system runs from 4.6 (mild steel, general purpose) through 8.8 (high tensile standard) to 12.9 (maximum strength alloy steel). The numbers encode strength directly: for grade 8.8, the first number × 100 = 800 MPa nominal tensile strength; both numbers × 10 = 640 MPa minimum yield strength.
For Australian trade and industrial work:
- Grade 4.6 — general hardware, non-structural, light-duty. Mild steel, no heat treatment.
- Grade 8.8 — the standard high-tensile grade. Minimum for structural steel connections under AS 4100. Suitable for machinery bases, flanges, motor mounts, and most industrial fastening.
- Grade 10.9 — higher clamping force, used in automotive powertrain, slip-critical structural connections, high-load applications. Do not hot-dip galvanise grade 10.9.
- Grade 12.9 — maximum strength. Cylinder heads, hydraulic equipment, precision clamping. Brittle under shock loading; handle and install with care.
Hex bolt finishes
Bolt finish determines corrosion resistance in service. Selecting the wrong finish is a common error — and in structural or outdoor applications, it is a maintenance problem that compounds over time.
| Finish | Also called | Corrosion resistance | Best for | Limitations |
|---|---|---|---|---|
| Plain / uncoated | Black, bare steel, plain finish | None — will rust immediately in wet conditions | Indoor, dry environments only; applications where a coating will be applied (paint, grease) | Not for outdoor use, wet environments, or any exposure to moisture |
| Zinc plated — clear | Electrozinc, zinc plate, zinc clear passivate | Moderate — 72–96 hours salt spray | Indoor and sheltered outdoor applications; general mechanical and structural in non-aggressive environments | Not suitable for marine, coastal splash zones, or chemical environments |
| Zinc plated — yellow | Yellow zinc, zinc yellow passivate, zinc yellow dichromate | Moderate-good — 120–200 hours salt spray | Similar to clear zinc with marginally better corrosion performance; common on grade 8.8 and 10.9 bolts | Same outdoor limitations as clear zinc |
| Hot-dip galvanised (HDG) | HDG, galvanised, gal bolt | High — 500–1,500+ hours salt spray depending on coating thickness | Outdoor structural steel, exposed fixings, agricultural, coastal (above splash zone) | Not suitable for grade 10.9 or 12.9 — pickling process before galvanising risks hydrogen embrittlement. Galvanised bolt requires galvanised nut (oversize to allow for coating thickness). |
| 304 stainless steel | A2 stainless, 304 SS | Good — atmospheric and mild environments | Food processing, general outdoor, moderate corrosion environments | Not for marine immersion, direct saltwater, or chlorinated water — use 316 SS instead |
| 316 stainless steel | A4 stainless, 316 SS, marine grade | Excellent — chloride and marine environments | Marine, coastal (including splash zones), swimming pools, chemical plant | Higher cost; not a structural substitute for grade 8.8 in AS 4100 connections — see bolt grade chart for full comparison |
| Black oxide / black phosphate | Black finish, black bolts | Low — provides rust inhibition only; typically oil-coated | Indoor, light corrosion protection; common finish on grade 12.9 (safe processing for high-hardness steel) | Not all black bolts are grade 12.9 — always read head markings. Not for outdoor use without additional protection. |
Bolt vs screw: the technical distinction
In precise fastener terminology, a bolt is a fastener designed to pass through clearance holes and be secured by a nut. A screw is a fastener that engages its own mating thread — either in a tapped hole or by creating its own thread in the material (as with a self-tapping screw or coach screw).
Applied to hex fasteners:
- A hex bolt (ISO 4014 / DIN 931 partial thread) passes through unthreaded clearance holes and is secured with a nut. The nut provides the clamping force.
- A hex set screw or hex cap screw (ISO 4017 / DIN 933 full thread) engages a tapped hole and provides clamping force without a nut. The term "hex set screw" is used on Australian product catalogues for fully-threaded hex fasteners regardless of application.
In practice, the terms "hex bolt" and "hex screw" are used interchangeably in most Australian trade contexts — and this is fine for general ordering purposes. The distinction matters when you are checking a specification drawing, verifying against an engineering standard, or selecting between partial-thread and full-thread products from a catalogue. On AIMS Industrial product listings, "hex bolt" generally refers to the partial-thread product and "hex set bolt" or "hex set screw" to the fully-threaded product.
For applications where a spanner cannot reach the head — counterbored holes, recessed mountings, machined assemblies — the equivalent fastener is the socket head cap screw (Allen bolt / DIN 912). Driven by an Allen key from above instead of a spanner from the side, socket head cap screws are stocked at higher property classes (Class 12.9 standard) and are the engineering default for precision joints. Choose hex bolts where side clearance and quick-release matter; choose socket head cap screws where compactness, recessed installation, or maximum strength matter. For the full head-shape comparison across hex, cap, button, truss, countersunk and other styles, see our Screw Head Types Guide.
Coach bolt (cup head bolt)
The coach bolt — formally called a cup head bolt in Australian and New Zealand standards (AS 1390) — is a distinctly different fastener from a hex bolt, designed specifically for timber and timber-to-steel applications. It is called a "carriage bolt" in North America and the UK.
The anatomy of a coach bolt distinguishes it immediately from a hex bolt:
- Cup (dome) head — a shallow, rounded head with no flat faces for a spanner. Once installed, the head cannot be driven or removed from the bolt side — there is nothing for a tool to grip.
- Square neck — directly beneath the head, a short square section. When the bolt is driven into a timber member, the square neck bites into the wood and prevents rotation as the nut is tightened from the other side. No tool is needed on the bolt head — the square neck locks it.
- Threaded shank — the remainder of the shank is threaded for its full length.
The installation method follows directly from this design: drill a clearance hole through both members, drive the bolt through from the smooth head side, apply a washer and nut on the thread side, and tighten the nut. The square neck locks into the timber as tightening begins, and the result is a tamper-resistant fixing — the head cannot be backed off without access to the nut side.
Common applications in Australia
Coach bolts are standard fasteners in:
- Timber decking — connecting deck boards to bearers, fixing handrails
- Pergolas and outdoor structures — connecting rafters, posts, and beams
- Fencing — connecting rails to posts, fixing gate hardware
- Playground equipment and public furniture — the tamper-resistant head limits vandalism
- Timber framing — connecting timber to steel or timber to timber in structural applications
The dominant sizes in Australian trade are M10 and M12, in lengths from 50mm to 200mm. Hot-dip galvanised finish (grade 4.6 UTS to AS 1390) is the standard for outdoor structural applications. Zinc-plated versions are available for sheltered indoor use. Stainless steel (316 SS) is specified for coastal or marine environments.
Hex flange bolt
A hex flange bolt (or flanged hex bolt) has a standard hexagonal head combined with an integrated washer-like flange at the base of the head. The flange is an integral part of the forging — it cannot be removed. Its purpose is to distribute the clamping load over a larger bearing area, eliminating the need for a separate washer in most applications.
Serrated vs plain flange
Two variants exist:
- Serrated (or serrated flange bolt) — the underside of the flange has radial serrations that bite into the mating surface when the bolt is tightened. The serrations lock against loosening under vibration — the bolt cannot rotate backward without overcoming the mechanical engagement of the serrations. This is the dominant type for automotive, plant equipment, and any application subject to vibration or cyclic loading.
- Plain flange bolt — the flange underside is smooth. Provides load distribution without the locking action. Used where the serrated version would damage a surface coating or where freedom of movement is required.
Hex flange bolts are extensively used in automotive and engine-related fastening (exhaust manifolds, engine covers, transmission housings), agricultural machinery, and general equipment where separate washers would be lost during assembly or servicing. In the AIMS Industrial range, hex flanged bolts are available in class 8.8 and 10.9, both zinc-plated and metric fine thread variants for demanding vibration applications.
Coach screw (hex head lag screw)
A coach screw is a heavy-duty threaded fastener with a hexagonal head and a coarse lag thread designed to cut into and grip timber or other soft materials directly — without a nut. It is not a bolt. The thread is a wood-screw type thread (coarser pitch, sharper thread form than a machine thread), and the screw is driven into a pre-drilled pilot hole by turning the hex head with a spanner or socket.
Coach screws are covered by AS/NZS 1393, which specifies mechanical requirements for screws with ISO hexagon heads for use in timber. They are also called "lag screws" or "lag bolts" in North American usage — these terms mean the same fastener.
Coach screw vs coach bolt — the confusion
These are the two most commonly confused fasteners in Australian timber construction and renovation work:
| Feature | Coach screw (hex head lag screw) | Coach bolt (cup head bolt) |
|---|---|---|
| Head shape | Hexagonal (flat faces for spanner) | Round dome (no flat faces) |
| Thread type | Lag/wood thread — coarse, for direct timber engagement | Machine thread — requires a nut |
| Nut required? | No — threads directly into timber | Yes — nut on back side of joint |
| Installation | Drill pilot hole, drive with spanner into timber | Drill clearance hole, insert bolt, apply nut from back |
| Best for | Timber-to-timber or timber-to-steel one-sided access; fence posts, joist hangers, pergola beams | Timber-to-timber through-bolting; structural connections requiring through-bolt clamping force |
| Removable from bolt side? | Yes — hex head can be driven both directions | No — round head cannot be gripped; access required from nut side |
The practical rule: if you have access from one side only and you are fixing into timber, use a coach screw. If you have access from both sides and need through-bolt clamping, use a coach bolt.
Hex bolt selection guide
The table below summarises which fastener type and grade to specify for common Australian trade and industrial applications. These are starting-point recommendations — always verify against design specifications and relevant standards where safety-critical connections are involved.
| Application | Fastener type | Grade | Finish | Notes |
|---|---|---|---|---|
| General machinery frames, guards, covers | Hex bolt (partial thread) | 8.8 | Zinc plate | 8.8 preferred over 4.6 where vibration is present |
| Structural steel connections (AS 4100) | Hex bolt or structural assembly (K0/K1) | 8.8 minimum | HDG or zinc | Grade 4.6 only in bearing joints per engineer; slip-critical joints: 8.8 or 10.9 |
| Motor flanges, base plates, mechanical mounts | Hex bolt (partial thread) | 8.8 | Zinc plate or plain | Partial thread to ensure shear plane in shank |
| Automotive powertrain fasteners | Hex bolt (partial or full as specified) | 10.9 (OEM spec) | Zinc or black | Always use OEM grade and torque — not negotiable |
| Tapped hole assembly (no nut) | Hex set screw (full thread) | 8.8 | Zinc plate | Full thread maximises thread engagement in tapped hole |
| Vibrating equipment, engine covers | Hex flange serrated bolt | 8.8 or 10.9 | Zinc | Serrated flange resists vibration loosening without separate locking washer |
| Timber decking, pergolas, outdoor structures | Coach bolt (cup head bolt) | 4.6 UTS (AS 1390) | HDG | M10 or M12 dominant; stainless for coastal/marine |
| Fence posts, joist hangers, one-sided timber fixing | Coach screw (lag screw) | 4.6 or 8.8 | HDG or zinc | Pilot hole required; size per AS/NZS 1393 |
| Food processing, mild corrosive environments | Hex bolt (partial or full) | A2-70 (304 SS) | 304 stainless | A4-70 (316 SS) if chloride exposure present |
| Coastal, marine, swimming pool | Hex bolt (partial or full) | A4-80 (316 SS) | 316 stainless | Do not use 304 SS in direct saltwater or chlorinated environments |
| Exposed outdoor structural — agricultural, civil | Hex bolt (partial thread) | 8.8 | HDG | HDG safe for 8.8; do not HDG grade 10.9 or 12.9 |
Australian standards for hex bolts
The key standards governing hex bolt specification in Australia are:
- AS/NZS 4291.1 — Mechanical properties of fasteners: bolts, screws and studs. The Australian and New Zealand adoption of ISO 898-1. Governs metric property classes 4.6 to 12.9.
- AS 1110 series — Dimensions and tolerances for metric hex bolts (AS 1110.1 for full thread, AS 1110.2 for partial thread). These establish the head dimensions, thread lengths, and tolerances that apply to AU-market hex bolts.
- AS 4100 — Steel Structures. The governing standard for structural steel connections in Australia. Specifies minimum fastener grades (8.8 for high-strength connections) and installation requirements. Bolt assemblies for AS 4100 structural connections are supplied as K0 or K1 assemblies (bolt + nut + washers).
- AS 1390 — Cup head bolts (coach bolts). Governs cup head bolt dimensions and grades.
- AS/NZS 1393 — Coach screws with ISO hexagon heads. Governs coach screw mechanical requirements for timber applications.
Frequently asked questions about hex bolts
What is the difference between a hex bolt and a hex screw?
Technically, a hex bolt passes through unthreaded clearance holes and is secured with a nut; a hex screw engages a tapped hole directly without a nut. In Australian practice, the terms are often used interchangeably. The practical distinction that matters for ordering: "hex bolt" typically refers to a partial-thread fastener (ISO 4014 / DIN 931) designed for through-bolting with a nut, while "hex set bolt" or "hex set screw" refers to a full-thread fastener (ISO 4017 / DIN 933) used in tapped holes or for full thread engagement. Check which you need before ordering.
What is the difference between partial thread and full thread hex bolts?
A partial-thread hex bolt (DIN 931 / ISO 4014) has an unthreaded shank section between the head and the threaded end. When correctly sized, the shear plane falls in the smooth shank — which provides full cross-sectional area and no stress concentrations. A full-thread bolt (DIN 933 / ISO 4017) has thread to the head, maximising thread engagement but placing thread roots (stress concentrations) at the shear plane. Partial thread is correct for structural joints, machinery, and flanges under shear or combined loading. Full thread is correct for tapped-hole assemblies and applications requiring maximum thread engagement.
What size spanner do I need for common metric hex bolts?
The spanner size matches the AF (across flats) dimension of the bolt head. Standard metric coarse: M8 = 13mm; M10 = 17mm; M12 = 19mm; M16 = 24mm; M20 = 30mm; M24 = 36mm; M30 = 46mm; M36 = 55mm. These are the same across ISO 4014 and ISO 4017 bolts of the same nominal size. For M6: 10mm. Always verify against the actual bolt when working on unfamiliar equipment — some imported equipment uses non-standard head dimensions.
What is the difference between a coach bolt and a hex bolt in Australia?
A coach bolt (formally "cup head bolt" per AS 1390) has a round dome head with no flat faces and a square neck below the head. It passes through both members and is secured with a nut — the square neck locks into timber to prevent rotation during tightening. It is used in timber-to-timber and timber-to-steel applications (decking, pergolas, fencing). A hex bolt has a six-flat hexagonal head that is tightened from the bolt side with a spanner or socket. Hex bolts are used in metal-to-metal joints, machinery, and structural steel connections where tool access to the bolt head is available.
What is the difference between a coach screw and a coach bolt?
A coach screw (also called a lag screw or lag bolt) has a hexagonal head and a coarse lag thread that cuts directly into timber — no nut required. It is driven from the hex head side with a spanner or socket into a pre-drilled pilot hole. A coach bolt has a dome head and machine thread, requires a nut, and passes through a clearance hole in both members. Use a coach screw when you have one-sided access and are fixing into timber. Use a coach bolt when you have access from both sides and need through-bolt clamping force. These are frequently confused but are not interchangeable.
What is a hex flange bolt and when should I use one?
A hex flange bolt has an integrated flange at the base of the head that acts as a built-in washer — distributing the clamping load over a larger bearing area. The serrated flange variant has radial serrations on the underside that bite into the mating surface, locking against vibration loosening without a separate locking washer. Use hex flange bolts where vibration loosening is a concern (automotive, engine-related, agricultural machinery) or where handling of separate washers during assembly is impractical. Plain flange bolts are used where serrations would damage a surface coating.
What is the difference between zinc plated and hot-dip galvanised hex bolts?
Zinc-plated bolts have a thin (5–15 micron) electroplated zinc coating applied by electrochemical deposition. This provides moderate corrosion resistance (typically 72–200 hours salt spray depending on clear or yellow passivate) and is suitable for indoor and sheltered outdoor use. Hot-dip galvanised (HDG) bolts are immersed in molten zinc, producing a thicker (45–85 micron) metallurgically bonded coating with substantially greater corrosion resistance — suited to exposed outdoor structural use, agriculture, and coastal environments above the splash zone. HDG bolts require oversize nuts (galvanised nuts) to accommodate the coating thickness. Critical restriction: do not hot-dip galvanise grade 10.9 or 12.9 bolts.
Can stainless steel hex bolts replace grade 8.8 high tensile bolts in structural connections?
Not in AS 4100 structural steel connections, which require carbon or alloy steel fasteners. Standard stainless grades (A2-70 at 700 MPa, A4-70 at 700 MPa) fall below grade 8.8's 800 MPa tensile strength. Even A2-80 or A4-80 at 800 MPa are not approved substitutes in AS 4100 connections, which also require certified yield strength, hardness, and installation torque specific to property class steel. For non-structural applications where corrosion resistance is the primary requirement, stainless is correct — but verify the strength class meets the load requirement before specifying.
What is DIN 931 and how is it different from DIN 933?
DIN 931 (now aligned with ISO 4014) is the German/international standard for partial-thread metric hex bolts. DIN 933 (ISO 4017) is the equivalent standard for full-thread hex bolts (hex set screws). Both designations still appear widely on product packaging and catalogue descriptions in Australia. DIN 931 = partial thread; DIN 933 = full thread. When a product is listed as "DIN 931" it has an unthreaded shank section; "DIN 933" has thread to the head. If the listing shows only a bolt size and grade with no DIN/ISO reference, confirm with the supplier whether it is partial or full thread before ordering.
How do I measure a hex bolt correctly?
A hex bolt is sized by nominal diameter and length. Diameter: measure across the shank (not the thread peaks) with calipers — this is the nominal metric size (e.g., 10mm = M10). Length: measure from the underside of the head to the tip of the bolt — the head is not included. A bolt marked M12 × 75 has a 12mm shank diameter and is 75mm long from under the head to the tip. To confirm thread pitch: count thread peaks over exactly 10mm of thread length with a rule — 6 peaks in 10mm = 1.5mm pitch (M10 coarse); 5–6 peaks in 10mm = 1.75mm pitch (M12 coarse).
What is a "cup head bolt" in Australia?
Cup head bolt is the formal Australian and New Zealand name, per AS 1390, for the fastener widely known as a coach bolt or carriage bolt. The name comes from the shallow cup-shaped (dome) head. All three names — cup head bolt, coach bolt, and carriage bolt — refer to the same fastener: dome head, square neck, machine thread, requires a nut. In trade practice, "coach bolt" is the most common term in Australian hardware and trade supply. "Cup head bolt" appears in formal specifications and AS 1390 product descriptions.
What thread pitch does an M12 coarse hex bolt have?
M12 coarse thread has a pitch of 1.75mm — meaning each full thread revolution advances the fastener 1.75mm. This is the standard pitch for M12 in ISO metric coarse thread series and is the default for general-purpose M12 hex bolts in Australia unless the product is specifically marked as fine thread (M12 × 1.25 or M12 × 1.5). To confirm: run a 19mm spanner across the M12 bolt head and count 5–6 thread peaks in 10mm of thread length.
What is the minimum bolt grade for structural steel connections in Australia?
AS 4100 (Steel Structures) specifies grade 8.8 as the minimum for high-strength structural connections in Australian steel structures. Grade 4.6 commercial bolts are permitted only in specific bearing-type connections as detailed by the structural engineer of record, and only where explicitly specified. Friction-type (slip-critical) connections require grade 8.8 or 10.9, installed to the specified proof load or snug-tight condition per AS 4100 requirements. Substituting a lower grade without engineering review is not permitted in AS 4100 structural work. See the AIMS Industrial Bolt Grade Chart for the full AS 4100 requirements and torque reference data.
Hex bolts and fasteners from AIMS Industrial
AIMS Industrial stocks the full range of metric and imperial hex bolts across grade 4.6, 8.8, and 10.9 — in zinc-plated, hot-dip galvanised, plain, and stainless steel (304 and 316) finishes. The range includes partial-thread hex bolts, full-thread hex set bolts, hex flange bolts (serrated and plain), cup head (coach) bolts to AS 1390, structural bolt assemblies (K0/K1) to AS 4100, coach screws, and assortment kits for trade and maintenance.
Browse bolts at AIMS Industrial
Related guides:
- Bolt Grade Chart: Metric, Imperial & High Tensile Markings Guide — complete grade table (4.6 to 12.9), head markings, and M6–M36 torque reference chart
- Types of Rivets: Pop Rivets, Blind Rivets, Solid Rivets & How to Choose
- Tap & Die Guide: Cutting and Repairing Threads