DIN 931 Guide for Selecting Hexagon Bolts with Semi-Thread Design

Standard Overview

The DIN 931 standard has been officially withdrawn by the German Institute for Standardization (DIN), and its technical content has been superseded by ISO 4014. In actual purchasing and engineering practice, DIN 931 is still widely used as a product name, but the latest design documents and procurement specifications should preferentially reference ISO 4014.

ISO 4014 and DIN 931 are interchangeable for most sizes, but M10, M12, M14, and M22 differ in width across flats (wrench size). This requires special attention when substituting parts or selecting tools. DIN 931 specifies partially threaded hex head bolts, covering metric thread sizes from M3 to M64, with lengths from 30 mm to 950 mm.

Key Technical Features

Partial Thread Design: The Engineering Significance of the Shank

The core difference between DIN 931 and DIN 933 (fully threaded) is the presence of an unthreaded shank. Between the head and the thread start, DIN 931 retains a smooth section of bar stock.

This shank is not a manufacturing defect—it serves clear engineering functions:

1. Shear load bearing: When the connection is subjected to transverse (shear) loads, the load is transferred through the shank rather than the threaded section. The thread root is a stress concentration point; the full cross-section of the shank resists shear force more efficiently. Mechanical design textbooks explicitly state: when a bolt is in shear, the critical location is the shank section, and the shank diameter must satisfy the shear strength requirement of the bar. The thread root cross-sectional area is smaller than the nominal shank area, so the shear capacity of the threaded section is significantly lower than that of the shank.

2. Precise alignment: The shank contacts the hole wall of the connected parts directly, providing more precise locating fit than threads. This is critical for structural connections requiring precision alignment (such as reamed hole bolt connections).

3. Reduced thread damage: During assembly, the shank enters the hole first to guide the bolt, preventing the threaded section from being scratched or damaged at the hole entrance.

4. Grip length matching: The shank length should be designed to ensure threads do not enter the connection shear zone. When the bolt carries transverse load, the threaded section stays outside the grip area while the shank sits exactly at the shear plane.

Thread Length Specifications

The thread length (b) of DIN 931 depends on the bolt's overall length (L) and thread diameter (d). Per the ISO 4014 dimensional system, thread length follows these rules:

Common size examples:

Property Grades and Materials

DIN 931 covers a complete range from low strength to ultra-high strength. Per ISO 898-1, mechanical properties by grade are as follows:

Grade 8.8 is the most common grade for structural connections. Sizes ≤M16 use low-carbon boron steel or medium-carbon steel; sizes >M16 require quenched and tempered medium-carbon steel.

Grade 10.9 uses medium-carbon alloy steel and must contain at least one alloying element (chromium ≥0.30%, nickel ≥0.30%, molybdenum ≥0.20%, vanadium ≥0.10%). It is quenched and tempered with Rockwell hardness controlled at HRC 32-39.

Dimensional Parameters (Common Sizes)

Per the ISO 4014/DIN 931 dimensional system:

Bolded sizes indicate where ISO and DIN width across flats differ.

Surface Finish Options

Zinc flake coating offers a key advantage: eliminating hydrogen embrittlement risk. For Grade 10.9 and above, the pickling and plating steps in electroplating can cause hydrogen atoms to diffuse into the steel, leading to delayed fracture. The British Constructional Steelwork Association (BCSA) explicitly states: Grade 8.8 is considered the highest strength grade that requires no special surface treatment protection; for Grade 10.9, electroplating is not recommended, and pickling is not permitted. Zinc flake coating uses a non-electrolytic process that fundamentally avoids this problem.

Thread Fit Issues with Hot-Dip Galvanizing

Hot-dip galvanized bolts undergo dimensional changes in the threads after coating (zinc thickness typically 65-85 µm). To ensure proper fit, a special thread tolerance system must be used.

Per ISO 10684 and fastener standard implementation guides:

Critical warning: Hot-dip galvanized bolts must not be used with standard 6H nuts. After galvanizing, the bolt's pitch diameter increases, leaving insufficient clearance with a standard nut—potentially causing assembly difficulty or connection failure. Only dedicated oversize-tapped nuts (6AZ or 6AX) may be used.

Application Areas

Steel Structure Connections

DIN 931 partially threaded bolts have a clear application logic in steel structure connections. For joints carrying shear loads, the shank sits exactly at the shear plane while the threaded section stays outside the grip area. This configuration allows the bolt's full cross-section to carry shear force rather than the stress-concentrated thread root.

Typical applications include steel frame beam-column connections, truss gusset plate connections, and bracing system connections.

Machinery Installation

In equipment anchor bolts, frame connections, and component assembly, the DIN 931 shank provides precise locating capability. This is especially important for connections requiring repeated disassembly or precision alignment.

Automotive and Heavy Vehicles

In chassis, suspension, and engine component assembly, high-strength DIN 931 bolts are used at critical connection points carrying dynamic loads. The 1040 MPa minimum tensile strength of Grade 10.9 ensures the connection maintains preload under vibration and impact.

Bridges and Infrastructure

In bridge structures, power transmission towers, and large infrastructure bolted connections, the partial thread design of DIN 931 combined with hot-dip galvanizing provides long-term structural safety and weather resistance.

Selection Guide

Summary

DIN 931 specifies partially threaded hex head bolts, and its core engineering value lies in the shank design—providing a full cross-section load path for shear, precise alignment for assembly, and accurate grip length matching for bolted connections. The standard has been superseded by ISO 4014, with M10/M12/M14/M22 differing in width across flats. Property grades cover 5.8 through 12.9, with Grade 10.9 using quenched and tempered alloy steel at Rockwell hardness HRC 32-39. Surface finishes from plain to hot-dip galvanized cover different corrosion environments. Zinc flake coating provides a hydrogen-embrittlement-free corrosion protection option for high-strength bolts, while hot-dip galvanized bolts must be paired with dedicated oversize-tapped nuts (6AZ/6AX). DIN 931 bolts are widely used in steel structure connections, machinery installation, automotive chassis, and bridge infrastructure, making them a foundational product in the metric high-strength fastener system.