Socket head cap screws — commonly known as Allen bolts — are among the most widely used fasteners in mechanical engineering, from industrial machinery and molds to high-precision components. This guide brings together the key standards, dimension tables, and selection guidance you need in one place.
What Is a Socket Head Cap Screw
A socket head cap screw features a cylindrical head with a hexagonal recess, driven by a hex (Allen) key or hex power bit. This design offers several advantages:
- High torque capacity with a much lower risk of cam-out compared to slotted or Phillips head screws
- Low-profile head that sits flush with or below the work surface, ideal for applications with limited clearance or where a clean finish is required
- High load-bearing capacity, as these screws are typically made from carbon or alloy steel and heat-treated for strength
DIN 912 vs. ISO 4762 — What's the Difference
A common question is whether DIN 912 and ISO 4762 refer to the same thing. The short answer: yes, in practice they describe essentially the same screw.
- DIN 912 is the original German standard (Deutsches Institut für Normung), in use for decades and still the name most familiar to the market.
- ISO 4762 is the international standard that superseded DIN 912 officially. The original DIN 912 has since been withdrawn, and Germany itself now uses DIN EN ISO 4762 as its official national standard.
- In practice, manufacturers and suppliers commonly label products as "DIN 912 / ISO 4762" together, since the dimensions are nearly identical across both. Engineers and procurement teams generally treat the two names as interchangeable for everyday use.
A note of caution: although the two standards are very close, small differences in tolerance can exist — particularly in minimum head diameter and minimum socket depth — between the older and current editions. For applications requiring precise fit, reference the latest ISO 4762 figures or the manufacturer's mill certificate directly.
How to Read the Dimension Symbols
Standard tables commonly use the following abbreviations:
| Symbol | Meaning |
|---|---|
| d | Nominal thread size |
| P | Thread pitch |
| dk | Head diameter |
| k | Head height |
| s | Hex socket size, measured across flats |
| t | Minimum socket depth |
A note on symbol conventions: some sources use different letters for the same dimension — for example, "A" instead of "dk," or "K" to denote socket depth instead of head height. Always check the legend of each specific table before applying the figures to real work.
Standard Dimension Table (ISO 4762 / DIN EN ISO 4762)
All values in millimeters(mm).
| Thread Size | Pitch (P) | Head Ø max (dk) | Head Ø min (dk) | Head Height (k) | Hex Socket (s) | Min. Socket Depth (t) |
|---|---|---|---|---|---|---|
| M3 | 0.50 | 5.50 | 5.32 | 3.00 | 2.5 | 1.3 |
| M4 | 0.70 | 7.00 | 6.78 | 4.00 | 3.0 | 2.0 |
| M5 | 0.80 | 8.50 | 8.28 | 5.00 | 4.0 | 2.5 |
| M6 | 1.00 | 10.00 | 9.78 | 6.00 | 5.0 | 3.0 |
| M8 | 1.25 | 13.00 | 12.73 | 8.00 | 6.0 | 4.0 |
| M10 | 1.50 | 16.00 | 15.73 | 10.00 | 8.0 | 5.0 |
| M12 | 1.75 | 18.00 | 17.73 | 12.00 | 10.0 | 6.0 |
| M16 | 2.00 | 24.00 | 23.67 | 16.00 | 14.0 | 8.0 |
| M20 | 2.50 | 30.00 | 29.67 | 20.00 | 17.0 | 10.0 |
| M24 | 3.00 | 36.00 | 35.31 | 24.00 | 19.0 | 12.0 |
Values based on the ISO 4762 (DIN EN ISO 4762) standard, provided for general reference. For production use requiring exact tolerances, always verify against the official standard or the manufacturer's certificate.
Choosing the Right Strength Class
Socket head cap screws are typically stamped with a two-part strength class marking. The first number, multiplied by 100, indicates the minimum tensile strength (in MPa); the second number indicates the yield point as a percentage of tensile strength.
- Class 8.8 — Medium strength (800 MPa), suitable for general-purpose applications
- Class 10.9 — High strength (1000 MPa), typically heat-treated alloy steel
- Class 12.9 — The highest strength class in the standard range (1200 MPa), suited to high-impact loads or heavy-duty machine components
Industrial Applications
Thanks to their high strength and flush-mounting head, DIN 912 / ISO 4762 socket head cap screws are widely used across industries — including machine tools and mold making, automotive and aerospace, robotics and automation, and plastics processing or granulating equipment where precision and high strength are both required.
This article is provided for general reference only. For design or production work requiring tight tolerances, always verify against the official standard or the manufacturer's certification.
Summary
DIN 912 and ISO 4762 are, in practical terms, the same standard — differing mainly in origin and current certification status. For technical documentation, reference dimensions per ISO 4762 as the primary source. Labeling products as "DIN 912 / ISO 4762" together remains an accepted and widely used industry convention. For guidance on screw sizing, strength class selection, or material choice for specific applications, our team is happy to help.
