DIN 931 and DIN 933 are two of the most frequently specified hex-head fastener designations in industrial purchasing. They look similar at a glance, but their thread coverage changes how they behave in a joint. The practical distinction is straightforward: DIN 931 normally refers to a partially threaded hex bolt, while DIN 933 normally refers to a fully threaded hex bolt. Choosing between them should be based on joint design, grip length, thread engagement and the applicable drawing—not habit.

This original guide helps buyers, engineers and maintenance teams compare DIN 931, DIN 933, DIN931 and DIN933 callouts. It also gives a quote-ready checklist for specifying metric hex bolts correctly.

DIN 931 vs DIN 933 at a glance

Feature DIN 931 DIN 933
Thread coverage Partially threaded; a plain shank remains between the head and thread Fully threaded to the underside of the head
Typical joint intent Clamped joints where the plain shank can pass through the assembled parts Short grip lengths, threaded holes, adjustment points or general fastening
Thread position Thread engagement is concentrated at the end of the bolt Threads are available along the whole nominal length
Common purchasing risk Wrong thread length for the stack-up Threads located in a shear or bearing plane when the design did not allow for it
Modern reference Often associated with ISO 4014 / EN ISO 4014 practice Often associated with ISO 4017 / EN ISO 4017 practice

Important: DIN 931 and DIN 933 remain common commercial search terms and drawing notes. For new projects, always confirm the governing customer, national or international standard and its required revision. A cross-reference does not make two products automatically interchangeable.

What the plain shank changes in a bolted joint

A DIN 931 hex bolt has an unthreaded section under the head before the threaded portion begins. In an appropriately designed joint, the plain shank can occupy the joined material thickness, often called the grip. This can improve fit-up and avoids placing threads where the design prefers a smooth bearing surface. It does not mean every partially threaded bolt is automatically stronger or better; the suitability depends on the dimensions, load case, clearance holes, material and engineering specification.

DIN 933 uses a thread that extends up to the head. That form is versatile where the bolt must engage a tapped component, where the grip is short, or where a specific all-threaded length is needed. It is also widely stocked, which can make it practical for general maintenance. However, the joint designer must still check whether threads are permitted in the intended shear plane and whether the nut or tapped hole has sufficient engagement.

Selection flow: start with the joint, not the catalogue

Question Why it matters Specification action
What is the assembled grip thickness? It determines whether a plain shank should cover the clamped parts. State the material stack-up and required bolt length.
Is the bolt used with a nut or a tapped hole? A nut connection and a threaded hole impose different engagement requirements. Specify the mating nut or thread depth and material.
Where is the shear plane? Some designs prefer a smooth shank in the bearing or shear region. Follow the drawing and engineering calculation; do not substitute by appearance.
What environment will the assembly see? Corrosion, temperature and vibration affect material and finish selection. State material, property class, coating and any locking method.
What inspection is required? Critical equipment may need traceability and documented verification. Include certificates, marking and inspection requirements in the PO.

DIN 931 applications: when a partially threaded bolt is useful

DIN 931 bolts are commonly considered for steel structures, flanged equipment frames, machinery joints and other through-bolted assemblies where a nut is used on the far side. The useful feature is not simply that it is “part thread”; it is that the available shank and thread lengths can be matched to the actual joint stack. A good design leaves enough threaded length for the nut, washer arrangement and required protrusion without forcing the thread transition into an unsuitable location.

DIN 933 applications: when full thread is the better fit

DIN 933 bolts can be appropriate for compact assemblies, mounting brackets, equipment covers, threaded bosses and many general fastening tasks. Because the thread is present along the length, the bolt can serve a wider range of short-thread-engagement situations. But thread availability is not a substitute for selecting the correct length. Excessively long bolts, insufficient engagement, damaged first threads or a mismatched nut can all reduce installation quality.

For a tapped hole, specify the nominal metric thread, pitch where needed, usable engagement depth and whether the hole is through or blind. For a nut-and-bolt assembly, specify the matching nut standard or product form, property class, finish and any required washer or locking element.

Material, property class and finish are separate decisions

The DIN 931 or DIN 933 designation describes a product form; it does not by itself complete a performance specification. A professional RFQ identifies the bolt material and property class, then adds the corrosion-protection or surface-treatment requirement. Carbon-steel, stainless-steel and alloy-steel bolts each suit different environments. Zinc plating, hot-dip galvanizing, mechanical galvanizing, zinc-flake systems and plain/oiled finishes all affect corrosion behaviour, appearance, thread fit and assembly friction.

Requirement to state Example of a clear description Why it prevents mistakes
Product form DIN 931 hex bolt, partially threaded Distinguishes the required thread coverage.
Size and length M16 × 90 mm, coarse pitch unless otherwise specified Defines the basic mating geometry.
Mechanical requirement Required property class per drawing or purchase specification Avoids treating geometry as a strength requirement.
Material and finish Specified steel/stainless grade with required coating or passivation Addresses service environment and corrosion risk.
Quality evidence Lot traceability, marking and certificate requirements where applicable Supports incoming inspection and maintenance records.

Five common buying errors

  1. Writing only “DIN933 bolt”. The supplier still needs diameter, length, material, property class, finish and quantity.
  2. Mixing metric and inch threads. A similar outside diameter does not prove thread compatibility.
  3. Assuming DIN 931 and DIN 933 are direct substitutes. The thread pattern changes the joint interface.
  4. Ignoring coating effects. A changed finish may affect corrosion protection and tightening behaviour.
  5. Omitting the mating component. Bolt, nut, washer and tapped-hole requirements should be reviewed as one assembly.

Quote-ready RFQ checklist for DIN 931 / DIN 933

Final takeaway

The fastest way to choose between DIN 931 and DIN 933 is to examine where the joint needs thread and where it benefits from a plain shank. DIN 931 supports designs that require a partially threaded hex bolt; DIN 933 offers full thread to the head for different connection needs. In both cases, a complete specification must also define size, material, property class, finish and mating components. That small amount of detail makes sourcing more reliable and reduces installation surprises.

Need help checking a bolt specification?

Songhai supports industrial buyers with flange, pipeline-component and custom connection-hardware enquiries. Send the drawing, service environment, standard, material, dimensions and quantity for a practical specification review before production.