- DIN 931 identifies a partially threaded metric hexagon-head bolt, not one universal material or strength level.
- Common engineering selections include nominal diameters such as M6, M8, M10, M12, M16, M20, and M24, subject to the applicable standard and project specification.
- Length, Thread Engagement, clearance, washer arrangement, strength class, corrosion protection, and tightening method must be specified together.
- DIN 931 bolts are suitable for steel structures, machinery, power-line hardware, brackets, frames, and maintenance assemblies when the joint design supports them.
- DIN 931 should not be treated as interchangeable with a fully threaded bolt, a high-strength structural bolt assembly, or an application-specific anchor system.
DIN 931 hex head bolts remain a practical reference for metric bolt standard selection because they define a familiar partially threaded form used across industrial assemblies. The ISO 4014 standard covers the corresponding hexagon-head bolt form with metric dimensions, while the NIST explanation of the SI system confirms the metric basis used for nominal diameters and lengths. This guide explains standard sizes, applications, selection limits, installation checks, and replacement decisions.
What DIN 931 Hex Head Bolts Mean
DIN 931 describes a hexagon-head bolt with a partially threaded shank and metric dimensions. The unthreaded portion can help the joint carry transverse loading through the shank rather than forcing all shear interaction into the threaded section, although the correct joint design remains decisive.
The standard number does not define every purchasing property. A complete specification normally includes the bolt standard, nominal diameter, length, thread pitch where non-coarse threading is required, material, property class, Surface Finish, and any inspection or packaging requirements.
DIN 931 is also a product-form reference rather than a complete structural design method. A bolt that matches the head and thread geometry may still be unsuitable if its strength class, coating, Corrosion Resistance, preload capability, or installation procedure does not match the joint.
Standard Sizes for the Metric Bolt Standard
Nominal diameter and length are the two primary size parameters for DIN 931 bolts. The nominal diameter refers to the approximate outside diameter of the threaded portion, while bolt length is measured from beneath the head to the end of the shank.
| Nominal size | Typical engineering role | Key selection question | Dimensional reference |
|---|---|---|---|
| M6 | Light brackets, guards, covers, and instrument supports | Can the joint maintain thread engagement without crushing the connected material? | ISO 4014 reference |
| M8 | Equipment frames, cable supports, and small structural attachments | Is the clearance hole and washer size compatible with the bracket? | ISO 4014 reference |
| M10 | Medium-duty machinery, support members, and line hardware | Does the joint require a larger bearing area or controlled preload? | ISO 4014 reference |
| M12 | Steelwork connections, mounting plates, and industrial assemblies | Will the unthreaded shank lie within the intended shear plane? | ISO 4014 reference |
| M16 | Heavy brackets, frames, tower components, and anchor-related assemblies | Are the connected plates, washers, and tightening method designed for the load? | ISO 4014 reference |
| M20 | Heavy steel structures and major equipment supports | Can the assembly control preload and prevent joint slip? | ISO 4014 reference |
| M24 | Large structural members and high-load industrial connections | Does the project require a dedicated structural bolt assembly instead? | ISO 4014 reference |
The size examples above are nominal metric diameters, not performance ratings. The applicable product standard and purchase specification should be checked before ordering because head dimensions, thread length, tolerances, and available combinations can vary with the standard edition and product grade.
How Bolt Length Is Selected
Correct bolt length leaves enough thread beyond the nut for complete engagement without creating unnecessary projection. The calculation should include the combined thickness of the joined parts, washers, any spacers, the nut height, and the required thread projection.
In a steel structure, length selection also determines whether the intended shear plane passes through the smooth shank or through the threaded portion. That distinction matters because the reduced root area of a thread can change the joint response. Designers should therefore compare the actual joint geometry with the governing design code rather than selecting length from appearance alone.
Applications in Steel Structure Fastening
DIN 931 bolts are used in steel structure fastening when the joint benefits from a partially threaded shank and a conventional hexagonal head. Typical applications include equipment supports, access platforms, guardrails, machine bases, frames, brackets, and secondary steelwork.
Power infrastructure creates a more demanding selection environment because a bolt may be exposed to moisture, pollution, vibration, temperature cycling, and galvanic contact with other metals. A suitable specification must address coating compatibility, washer use, tightening access, inspection, and replacement availability. For primary tower or connection systems, the project specification may require a dedicated structural bolt assembly rather than a general-purpose DIN-form bolt.
Mining and tunneling equipment often combines vibration, impact, abrasive dust, and difficult maintenance access. In these conditions, the bolt size alone does not establish reliability. Joint stiffness, locking strategy, corrosion protection, fatigue exposure, and the ability to inspect or retighten the connection are equally important.
Communication and cable-support hardware generally requires compact, repeatable fastening with adequate resistance to outdoor exposure. The bolt should be selected with its clamp plates, saddles, washers, and support members as a complete system. A nominally compatible bolt can still fail to deliver the intended clamp if the washer diameter or mating surface is unsuitable.
DIN 931 Versus Fully Threaded and Structural Bolts
The main functional difference is the location of the thread along the shank. DIN 931 bolts are partially threaded, whereas a fully threaded bolt places threads along nearly the entire usable length.
| Fastener form | Thread arrangement | Useful advantage | Important limitation |
|---|---|---|---|
| DIN 931 or ISO 4014 form | Partial thread | Allows a smooth shank in selected shear or bearing zones | Available thread length must suit the grip length |
| Fully threaded hex bolt | Thread over most of the shank | Flexible engagement for varied grip thicknesses and tapped holes | May place threads in a shear plane or reduce bearing behavior |
| Structural bolt assembly | Defined bolt, nut, and washer system | Designed for specified structural installation and inspection rules | May not be interchangeable with a general DIN-form bolt |
| Stud or anchor rod | Threaded ends or continuous thread | Useful where access, embedment, or permanent anchorage controls the design | Requires a different installation and load-transfer assessment |
Interchangeability must be checked at the system level. Matching thread diameter and pitch is not enough; head dimensions, nut compatibility, washer dimensions, strength class, coating thickness, thread length, and governing design requirements must also match.
Material, Strength Class, and Surface Protection
Material and property class determine whether a bolt can meet the required tensile, shear, preload, and environmental demands. Carbon steel is common for general industrial work, while stainless steel or specialized coatings may be selected for corrosion exposure, electrical compatibility, or maintenance constraints.
Property classes such as 4.6, 8.8, and 10.9 identify mechanical performance categories under the relevant fastener property standard; they should not be confused with the DIN product-form number. The same DIN 931 geometry may be supplied in different materials or classes, so the purchase order must state the complete requirement.
Surface treatment should be selected with the environment and mating materials in mind. Zinc-based coatings can provide practical atmospheric protection, but coating thickness can influence thread fit and hydrogen-related risks in certain high-strength steels. Hot-dip galvanizing, stainless steel, and other finishes each require compatibility checks for the full assembly.
For outdoor power or steelwork applications, corrosion design should consider water traps, drainage, crevice exposure, coating damage during installation, and contact between dissimilar metals. A corrosion-resistant bolt cannot compensate for an unprotected joint interface or poor drainage.
Installation and Inspection Checklist
Reliable performance depends more on joint preparation and installation control than on the bolt label alone. Before installation, confirm the following:
- Verify the standard, diameter, length, property class, material, finish, nut, and washer combination against the drawing or purchase specification.
- Inspect threads, bearing faces, coatings, and bolt identification for damage or contamination.
- Confirm that holes are aligned without forcing the bolt through by hammering or enlarging the hole during assembly.
- Check that the washer bears fully on the connected surface and that the nut receives complete thread engagement.
- Use the tightening method and torque or preload requirement specified by the responsible design authority.
- Record lot information and inspection results when the connection is safety-critical or exposed to difficult maintenance conditions.
Torque alone is not a universal measure of preload because friction varies with lubrication, coating, surface condition, thread damage, and washer behavior. Where preload is critical, the project should define the accepted installation method and verification process rather than relying on an assumed torque value.
Common Selection Errors
The most frequent error is choosing a bolt from diameter alone. A sound selection also evaluates grip length, load direction, hole condition, joint stiffness, corrosion exposure, access for tools, and future maintenance.
- Using a partially threaded bolt when the available thread does not reach the nut.
- Putting the threaded portion in a critical shear plane without checking the design basis.
- Replacing a specified structural assembly with a visually similar general-purpose bolt.
- Combining a high-strength bolt with an incompatible nut or washer.
- Ignoring coating compatibility, especially where dissimilar metals or outdoor exposure are present.
- Assuming a larger diameter automatically solves slip, fatigue, or joint loosening.
How to Specify DIN 931 Hex Head Bolts for Procurement
A procurement-ready description should remove ambiguity between standard geometry and application performance. A practical specification identifies the product form, nominal diameter, length, thread requirement, material, property class, surface treatment, nut and washer requirements, inspection documents, packaging, and delivery conditions.
| Specification item | Example of the information required | Why it matters |
|---|---|---|
| Product form | DIN 931 or applicable ISO equivalent | Controls head and partial-thread geometry |
| Nominal size | M6 through M24 examples | Defines the basic metric diameter family |
| Length | Project-specific measured length | Controls grip, engagement, and projection |
| Strength and material | Required property class and steel type | Matches design loads and service conditions |
| Finish | Specified coating or corrosion-resistant material | Addresses environment and compatibility |
| Assembly components | Compatible nut and washer set | Supports load transfer and installation consistency |
For engineered orders, the supplier should review drawings, application loads, environmental conditions, and inspection requirements before confirming an equivalent. This is particularly important when standard fasteners are used alongside forged, stamped, machined, or power-line components.
FAQ
Are DIN 931 bolts metric?
Yes. DIN 931 is a metric product-form standard for hexagon-head bolts with a partially threaded shank. The nominal diameter and length are expressed in millimeters, while the exact thread details depend on the applicable specification.
What is the difference between DIN 931 and DIN 933?
DIN 931 identifies a partially threaded hexagon-head bolt, while DIN 933 traditionally identifies a fully threaded hexagon-head bolt. The correct choice depends on grip length, shear-plane location, tapped-hole depth, and the design requirement.
Can DIN 931 be replaced by ISO 4014?
Often, ISO 4014 is the current comparison for the same general partially threaded hexagon-head form. Replacement should still be confirmed by comparing dimensions, tolerances, material, property class, coating, and project approval requirements.
Which DIN 931 size is used for steel structures?
There is no universal size for all steel structures. M12, M16, M20, and M24 are common nominal diameter examples in industrial work, but the required size must come from the joint design, applied loads, plate thickness, hole arrangement, and governing code.
Are DIN 931 bolts suitable for outdoor power-line hardware?
They may be suitable for selected brackets, supports, and assemblies when the strength, coating, washer arrangement, and corrosion environment are compatible. Primary line hardware may require an application-specific fastener or complete connection system.
Does a larger bolt always provide a stronger connection?
No. Connection strength can be limited by bolt shear, plate bearing, net-section rupture, pull-out, slip, fatigue, hole quality, or joint eccentricity. Increasing diameter without redesigning the connected parts may not address the governing failure mode.
What should be checked before ordering DIN 931 bolts?
Confirm the standard, diameter, length, thread arrangement, material, property class, coating, compatible nut and washer, inspection documentation, packaging, and delivery requirements. For critical applications, provide the assembly drawing and service environment to the supplier.
About the Supplier
Weiye operates as an integrated industrial component supplier covering fastening, forging, stamping, machining, heat treatment, and material testing. Its engineering-oriented product scope supports power infrastructure, steel structures, mining, tunneling, communications, renewable-energy infrastructure, and customized connection systems. Buyers can use this capability to coordinate component design, manufacturing, inspection, procurement, and logistics through one industrial supply channel. Contact the engineering team to discuss a DIN-form fastener requirement or a complete fastening solution.