The DIN912 Hex Socket Head Cap Screw — commonly called an Allen bolt or socket head cap screw — is one of the most widely used threaded fasteners in precision engineering. Its defining characteristic is a cylindrical head with an internal hexagonal (Allen) recess, driven by a matching Allen key or hex bit. This geometry allows application of high torque in confined spaces where a conventional wrench could never reach.
First standardized by the Deutsches Institut für Normung (German Institute for Standardization), DIN912 has been superseded internationally by ISO 4762, yet the DIN912 designation remains the dominant commercial reference worldwide. At Global Tuyue, we supply DIN912 compliant fasteners in 304 and 316 stainless steel, carbon steel, and alloy steel across a full metric size range — from M2 through M30.
Understanding the standards landscape helps engineers source correctly and write purchase orders without ambiguity.
In everyday procurement, "DIN912" and "ISO 4762" are interchangeable for metric dimensions. However, property class marking, thread pitch, and surface treatment must be specified separately. Always confirm the standard, material grade, and coating when issuing RFQs to avoid dimensional or mechanical surprises on the production floor.
The cylindrical head of a DIN912 screw is precisely sized: dk ≈ 1.5 × d and k ≈ d for most metric sizes. This compact, high-profile head distinguishes it from countersunk (flat) and button-head variants. The generous head-to-shank junction incorporates a fillet radius, distributing clamp load and reducing stress concentration — critical in fatigue-sensitive assemblies.
The socket width across flats (s) is standardized per DIN 912 / ISO 4762. For example, an M8 screw uses a 6 mm hex key. Insufficient socket engagement is a leading cause of key cam-out and head rounding — always use calibrated hex keys with the correct tolerance fit.
The workhorse grade — 18% chromium, 8% nickel austenitic alloy (AISI 304, ISO designation A2). It offers reliable corrosion resistance in most indoor and mild outdoor environments, good formability, and non-magnetic properties after annealing. Tensile strength ~515 MPa; yield ~205 MPa. Suitable for electronics, food processing, general machinery, and construction hardware.
Adding 2–3% molybdenum to the 18-8 alloy (AISI 316, ISO designation A4) dramatically improves resistance to chloride pitting and crevice corrosion. Essential for marine hardware, chemical processing, coastal construction, and pharmaceutical equipment. Tensile strength is comparable to 304 (~515 MPa), but the corrosion protection premium justifies the cost difference in aggressive environments.
Carbon and alloy steel DIN912 bolts are the go-to choice when maximum clamping force is required. Grade 12.9 (alloy steel, quenched and tempered) delivers ~1220 MPa ultimate tensile strength — more than double that of stainless — enabling smaller-diameter fasteners to carry equivalent loads. Common in automotive, heavy machinery, and tooling jigs. They require surface protection (zinc plating, black oxide, or Dacromet) to prevent corrosion.
The following table lists standard DIN912 / ISO 4762 nominal dimensions for the most common metric sizes. All values in millimetres.
| Size | Thread Pitch (mm) | Head Ø dk (nom.) | Head Height k | Hex Key s | Socket Depth t (min) | Common Lengths L |
|---|---|---|---|---|---|---|
| M3 | 0.50 | 5.5 | 3.0 | 2.5 | 1.3 | 6–30 |
| M4 | 0.70 | 7.0 | 4.0 | 3.0 | 2.0 | 8–40 |
| M5 | 0.80 | 8.5 | 5.0 | 4.0 | 2.5 | 8–50 |
| M6 | 1.00 | 10.0 | 6.0 | 5.0 | 3.0 | 10–60 |
| M8 | 1.25 | 13.0 | 8.0 | 6.0 | 4.0 | 12–80 |
| M10 | 1.50 | 16.0 | 10.0 | 8.0 | 5.0 | 16–100 |
| M12 | 1.75 | 18.0 | 12.0 | 10.0 | 6.0 | 20–120 |
| M16 | 2.00 | 24.0 | 16.0 | 14.0 | 8.0 | 25–160 |
| M20 | 2.50 | 30.0 | 20.0 | 17.0 | 10.0 | 30–200 |
| M24 | 3.00 | 36.0 | 24.0 | 19.0 | 12.0 | 40–200 |
Source: ISO 4762:2004 / DIN 912:2014-08 dimensional tables. All tolerances per ISO 4759-1, product grade A.
Achieving the correct clamp load is the primary goal of tightening a DIN912 fastener. The widely used torque formula is:
T = K × d × F
Where T is tightening torque (N·m), K is the dimensionless nut factor (≈0.20 for dry, uncoated steel; ≈0.15 for lubricated), d is the nominal bolt diameter (m), and F is the desired preload (N).
| Size | Torque (N·m) — Dry | Torque (N·m) — Lubricated | Preload F (kN) |
|---|---|---|---|
| M6 | 10 | 8 | 8.1 |
| M8 | 25 | 20 | 19.4 |
| M10 | 49 | 39 | 30.9 |
| M12 | 86 | 68 | 44.8 |
| M16 | 210 | 168 | 82.4 |
| M20 | 410 | 328 | 128.0 |
Values are approximate and assume 70% utilization of proof load. Always consult your joint design engineer for safety-critical applications and use calibrated torque wrenches.
The Hex Socket Round Head Bolt (DIN7380) — also marketed as a button head cap screw — pairs the same Allen drive with a low-profile, dome-shaped head. The comparison table below summarizes where each type excels:
| Feature | DIN912 (Cylindrical) | DIN7380 (Round/Button) |
|---|---|---|
| Head height | ≈ 1×d (tall) | ≈ 0.5×d (low-profile) |
| Bearing surface area | Larger | Smaller |
| Max recommended torque | High (full property class) | Reduced (~60–70% of DIN912) |
| Aesthetics | Industrial / precision | Clean / consumer-friendly |
| Countersink needed? | No (surface-mount) | No (dome protrudes slightly) |
| Typical applications | Machine frames, tooling, structural | Panels, enclosures, bicycles, furniture |
Also see our related products: Hex Socket Bolt DIN912 and Hex Socket Round Head Machine Screw.
