A Complete Guide to the Differences and Applications of DIN 931, DIN 933, and ISO 4014

A Complete Guide to the Differences and Applications of DIN 931, DIN 933, and ISO 4014

Author: QBH Fastener Technical Team
Background: Our core team members have over 10 years of experience in the fastener industry. We are well‑versed in international standards such as DIN, ISO, ASTM, and GB, and have provided selection support for clients in wind power, rail transportation, heavy machinery, and other sectors.

Introduction
In mechanical manufacturing, steel structure, and industrial assembly, choosing the right bolt standard is critical. Many buyers and engineers often confuse DIN 931 with DIN 933, or find the relationship between DIN and ISO standards perplexing. In this article, we address these frequently asked questions based on our years of field experience, offering professional and clear answers to help you avoid selection mistakes.

What is DIN 931 and where is it mainly used?

DIN 931 is officially titled “Hexagon head bolts – Partially threaded.” Its defining characteristic is that the shank is not fully threaded; it consists of a partially threaded section plus a plain shank (unthreaded portion) .

Main applications:

Connections subject to shear forces: The plain shank helps position the joint and bear transverse shear loads, preventing stress concentration on the threaded portion.

Joining thick workpieces: When the total clamping thickness is large and the bolt needs to pass through thick plates, the plain shank provides better guidance.

High‑precision mounting: Commonly used for mounting motor bases and positioning heavy machinery.

Real‑world case:

A port machinery client originally used fully threaded bolts for the slewing mechanism of a quayside crane. Frequent vibrations caused bolt fractures. After an on‑site inspection, we recommended switching to DIN 931 partially threaded bolts. By utilizing the plain shank to absorb shear forces, the failure rate dropped by over 90%.

What is the difference between DIN 931 and DIN 933 (fully threaded)? Which one should I choose?

This is the most common question. Both are hex head bolts, but they differ fundamentally in design and application.

Selection advice:

If your connection is subject to vibration or shear forces, or requires precise positioning, choose DIN 931.

For simple through‑bolting or when space is limited and a nut is required, DIN 933 is the more versatile option.

What is the relationship between DIN 931 and ISO 4014? Are they interchangeable?

ISO 4014 is the International Organization for Standardization (ISO) equivalent of DIN 931. For most sizes (especially the common range from M1.6 to M39), they are interchangeable in terms of mechanical properties, width across flats, and thread tolerances.

However, one important exception:
For specific sizes such as M10, M12, M14, and M22, there is a slight difference in the width across flats (wrench size) between the DIN and ISO standards.

DIN 931: M10 typically has a width across flats of 17 mm.

ISO 4014: M10 typically has a width across flats of 16 mm.

The above differences are verified against DIN 931‑1:1987 and ISO 4014:2011. If you need the original standards, please contact us.

How do I choose the property class for DIN 931? What’s the difference between 8.8 and 10.9?

DIN 931 is commonly available in property classes 8.8, 10.9, and 12.9.

Grade 8.8: The entry‑level high‑strength grade. Suitable for general steel structures and ordinary mechanical assembly. It offers the best cost‑effectiveness and is the most widely used.

Grade 10.9 / 12.9: Ultra‑high‑strength grades. Suitable for critical applications demanding high preload, such as automotive, heavy machinery, and wind power equipment.

Note: High‑strength bolts (grade 10.9 and above) require attention to hydrogen embrittlement prevention during surface finishing (e.g., Dacromet or hot‑dip galvanizing). If your application is in coastal or corrosive environments, please let us know the required salt spray test duration.

What surface finishes are available for DIN 931? How do I prevent rust?

We recommend the following common surface treatments based on the application environment:

1. Plain / Black oxide: For indoor, dry environments. Low corrosion resistance but the most economical. Usually coated with anti‑rust oil.

2. Zinc plating (blue‑white or yellow): The most common medium‑level corrosion protection. Good appearance and meets standard industrial requirements.

3. Hot‑dip galvanizing: A thick coating with high corrosion resistance, suitable for outdoor use and power transmission towers. Note: Hot‑dip galvanizing can affect thread fit; thread size allowances (e.g., tapping after galvanizing) are typically required.

4. Dacromet (zinc‑aluminum coating): High corrosion resistance (salt spray tests can exceed 1000 hours) with no risk of hydrogen embrittlement. It is the preferred choice for automotive and high‑end equipment.

About Us & Quality Assurance

QBH Fastener specializes in the R&D and supply of high‑strength fasteners. We are ISO 9001:2015 certified and manufacture products strictly in accordance with DIN, ISO, ASTM, GB, and other standards. Every batch comes with material certificates and inspection reports for full traceability.

Technical team: Average 10+ years of industry experience; free selection consulting available

Testing capabilities: Equipped with spectrometers, hardness testers, salt spray chambers, and more

Success stories: Serving clients in wind power, construction machinery, rail transportation, and beyond