In engineering procurement, hex bolts are among the highest-volume fasteners, and DIN 931 and DIN 933 are the two German industrial standards most frequently referenced — and most easily confused — in the international market. This article breaks down the differences in thread length, load characteristics, and application scenarios to help procurement engineers select quickly.
1. Core Difference: Does the Thread Cover the Full Length?
DIN 931 and DIN 933 look almost identical — both are hex-head, metric thread, with the same strength grade system. The single, most critical difference is how much of the shank is threaded:
| Standard | Name | Thread Feature | Typical Use |
|---|---|---|---|
| DIN 933 | Full-Thread Hex Bolt | Thread runs the entire length of the shank | Thin-plate connections, nut can engage anywhere |
| DIN 931 | Half-Thread Hex Bolt | Shank retains a smooth (unthreaded) section | Thick-plate connections, structural parts under shear load |
Memory tip: Think of DIN 933’s “3” as thread coverage of “3” — i.e. the full/entire length; DIN 931 retains a smooth “shank” section, technically known as “half-thread”.
2. Why Does the Smooth Shank Matter?
The smooth shank on DIN 931 isn’t a cost-cutting shortcut — it serves clear engineering purposes:
- Higher shear strength: The smooth section has a full, uninterrupted cylindrical cross-section with no thread-related stress concentration, so it performs better under shear load (forces perpendicular to the bolt axis).
- Precise alignment: Where the bolt needs a tight fit with the hole (e.g. flange connections, machine assembly), the smooth shank acts like a dowel pin, reducing relative movement between connected parts.
- Less thread wear: When the clamped material is thick and the thread would otherwise sit fully inside the hole, prolonged vibration can seize or wear the threads — the smooth shank design avoids this.
3. Choosing Thread Length Based on Material Thickness
The thread length (b) on DIN 931 follows an internationally standardized formula, based on total bolt length (L) and nominal diameter (d):
- When L ≤ 125mm: b = 2d + 6mm
- When 125mm < L ≤ 200mm: b = 2d + 12mm
- When L > 200mm: b = 2d + 25mm
When ordering, we recommend leaving at least 2–3 exposed thread turns beyond the total thickness of the clamped material, to ensure the nut can be fully tightened.
4. Strength Grade and Material Selection
Both DIN 931 and DIN 933 are available in the following strength grades:
| Grade | Tensile Strength | Typical Application |
|---|---|---|
| 4.8 | ≥400 MPa | Light-load connections, non-load-bearing structures |
| 8.8 | ≥800 MPa | General machinery, structural steel (most common) |
| 10.9 | ≥1000 MPa | High-strength structural connections, heavy machinery |
| 12.9 | ≥1200 MPa | Aerospace, high-end equipment (use with caution — hydrogen embrittlement risk) |
5. Purchasing Recommendations
- For connection points bearing shear loads — steel bridges, industrial buildings — prioritize DIN 931 half-thread bolts.
- For thin-plate flanges, equipment housings, and other connections without significant shear requirements, DIN 933 full-thread bolts offer better value and more universal stock availability.
- When requesting quotes from suppliers, always specify the standard (DIN 931/933), size (e.g. M16×80), strength grade (e.g. 8.8), and surface treatment (e.g. hot-dip galvanized) to avoid receiving parts that don’t meet your engineering requirements due to standard confusion.
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