If you spend any time in construction, mechanical engineering, or manufacturing, you've undoubtedly handled a DIN 931 bolt. It is one of the most ubiquitous and trusted fasteners in the industrial world.
But what exactly does "DIN 931" mean, and why is it specified on so many engineering drawings instead of other hex bolts?
Welcome back to Fastener Insight. Today, we are breaking down everything you need to know about the DIN 931 hex head cap screw — from its key characteristics to when you should choose it over its fully threaded cousins.
What is a DIN 931 Bolt?
"DIN" stands for Deutsches Institut für Normung (the German Institute for Standardization). While many DIN standards are being transitioned to ISO (International Organization for Standardization) equivalents, DIN 931 remains the most common industry shorthand for a very specific type of fastener: a hexagon head cap screw with a partial thread.
The defining feature of a DIN 931 bolt is its shank. Unlike fully threaded bolts, the DIN 931 features an unthreaded portion beneath the head (often called the grip length) before the threading begins.
(Note: The internationally recognized ISO equivalent for DIN 931 is ISO 4014).
Why Use a Partially Threaded Bolt?
You might wonder why you wouldn't just use a fully threaded bolt for everything to ensure maximum grip. The answer comes down to shear strength.
When two pieces of material are fastened together and subjected to sideways forces (shear forces), those forces slice across the bolt. Threads are essentially tiny grooves cut into the metal, which act as stress concentrators and reduce the bolt's overall cross-sectional area.
If the shear plane falls on the threaded portion of a bolt, it is significantly more likely to fail. The unthreaded shank of a DIN 931 bolt provides a solid, uniform cylinder of metal to absorb those shear forces, making it vastly superior for load-bearing joints where sideways movement is a factor.
Key Specifications and Materials
Because it is a globally recognized standard, you can find DIN 931 bolts in a massive variety of sizes and materials to suit almost any environment.
- Size Range: Typically available in metric sizes from M1.6 all the way up to heavy-duty M64.
- Steel Property Classes: Most commonly found in high-tensile grades like 8.8, 10.9, and 12.9.
- Stainless Steel: For corrosive environments, they are widely manufactured in A2 (304) and A4 (316) stainless steel.
- Finishes: Available in plain (bare metal), zinc-plated (for mild corrosion resistance), and hot-dip galvanized (for harsh outdoor exposure).
The Classic Matchup: DIN 931 vs. DIN 933
The most common point of confusion in the fastener world is the difference between DIN 931 and DIN 933. They look nearly identical from the top down, but their applications are very different.
| Feature | DIN 931 | DIN 933 |
|---|---|---|
| Thread Type | Partial Thread | Full Thread |
| Best For | Shear strength and precise alignment | Tapped holes and high-tension grip |
| ISO Equivalent | ISO 4014 | ISO 4017 |
| Shank | Unthreaded grip area below head | Threaded all the way to the head |
If you are threading a bolt directly into a tapped hole rather than using a nut, or if you need maximum tensioning power across a short distance, DIN 933 is your go-to. If you are bolting two plates together and securing them with a nut on the other side, the smooth shank of the DIN 931 is usually the safer engineering choice.
Common Applications
Because of its high shear strength and reliable standardization, DIN 931 bolts are the backbone of heavy industry. You will find them in:
- Structural Steelwork: Connecting beams and columns where shear forces are immense.
- Automotive & Heavy Machinery: Engine mounts, suspension components, and chassis assembly.
- Pumps and Valves: Securing heavy flanges where precise alignment and strength are required.
The Bottom Line
The DIN 931 hex bolt isn't just a basic piece of hardware; it is a meticulously standardized component designed to keep our infrastructure, vehicles, and machinery from shearing apart.
Knowing when to specify a partial thread (DIN 931) versus a full thread (DIN 933) is a fundamental skill for any engineer or procurement specialist.