Our metric socket head bolts, manufactured to DIN 912 and ISO 4762, are high-strength cap screws driven by an internal hex (allen) key. Their cylindrical head and recessed drive make them the go-to fastener for machine tooling, jigs, dies, and precision assemblies where clamping force and a clean, low-clearance profile both matter.

Socket Head Bolts - Metric Sizes

What Is a Socket Head Bolt?

A socket head bolt often called a socket head cap screw, allen bolt, or SHCS is a threaded fastener with a cylindrical head and an internal hexagonal recess that is turned with an allen (hex) key rather than an external wrench. Because the drive is internal, the tool engages fully inside the head, allowing very high torque to be applied without the rounding or cam-out common to external drives. This makes socket head bolts one of the most reliable high-clamp-force fasteners available in metric hardware.

Manufactured to DIN 912 and its international equivalent ISO 4762, socket head cap screws are produced from cold-forged, heat-treated alloy steel to deliver tensile strengths far above general-purpose bolts. Their compact head diameter and the ability to sit in a counterbored hole flush with a surface make them essential wherever mechanical designers need maximum strength in minimal space  machine frames, tooling plates, hydraulic manifolds, robotics, and precision jigs and fixtures.

Key Features and Benefits

• High Clamping Force: Class 12.9 alloy steel delivers exceptional tensile and yield strength, enabling tight, vibration-resistant joints in loaded machinery.
• Internal Hex Drive: The recessed allen socket engages the full depth of the tool, transmitting high torque with minimal cam-out or head damage.
• Low-Clearance Profile: The compact cylindrical head fits counterbored holes, sitting flush or recessed for a clean, snag-free surface.
• Precision Manufacture: Cold-forged and rolled threads produce accurate, repeatable fits ideal for tooling and automated assembly.
• Space Efficiency: Smaller head diameter than hex bolts allows fasteners to be placed closer together and in tight recesses.

Materials and Finishes

• Alloy Steel Class 12.9 (Black Oxide): The industry-standard high-strength option for tooling, dies, and heavily loaded machine joints.
• Alloy Steel Class 10.9 (Zinc Plated): High strength with added corrosion resistance for general engineering.
• Stainless Steel A2 / A4: Corrosion-resistant grades for food processing, marine, and outdoor equipment.

Standards Compliance

• DIN 912 / ISO 4762  Hexagon socket head cap screws.
• ISO 898-1  Mechanical property classes for carbon and alloy steel bolts.
• DIN 6912  Low-head socket cap screws for reduced clearance.

Common Applications and Industries

Socket head cap screws are specified across virtually every precision engineering sector. In machine tool and tooling manufacture they fasten die sets, jig plates, and fixture bases, where high clamp force and a counterbored, flush profile keep working surfaces clear. In hydraulics and pneumatics they secure valve blocks and manifolds against high internal pressure. Robotics, automation, and CNC equipment rely on them for compact, high-strength joints that can be placed close together in tight housings.

The automotive and motorsport sectors use socket head bolts in engines, gearboxes, and chassis components where weight, strength, and access all matter, while the mould-making industry depends on them to clamp tooling that must withstand repeated high-pressure cycles. Their combination of strength, compactness, and clean appearance also makes them popular in premium consumer products, medical devices, and architectural metalwork where fasteners are visible.

How to Choose the Right Socket Head Bolt

Start with the load: heavily loaded or safety-critical joints call for Class 12.9 alloy steel, while general assembly may use Class 10.9 or stainless. Next consider the environment  indoor machinery suits black-oxide alloy steel, whereas wet, marine, or hygienic settings need A2 or A4 stainless. Then confirm the thread (coarse for general use, fine for precise adjustment or thin-walled components) and the required length, allowing for the grip and thread engagement your joint needs.

Finally, plan the hole. A counterbore lets the head sit flush or recessed for a clean finish, and the counterbore diameter must clear the head. For very limited clearance, a low-head socket cap screw (DIN 6912) reduces head height at the cost of some torque capacity. Matching the hex key size to the socket and tightening to the correct torque completes a reliable, reusable joint.

Size Range and Availability

Available in M2 through M48 in both metric coarse and metric fine thread, in a wide range of lengths from short set screws to long structural bolts.

Why Buy From Metric Specialties

As dedicated metric fastener specialists, we hold socket head bolts in the sizes, grades, and finishes that engineers actually need, backed by clear DIN and ISO standard references so you can specify with confidence. Every item is sourced to recognised standards and quality-controlled, giving you consistent thread fit, strength, and corrosion performance across repeat orders.

Whether you need a single item for a repair or bulk quantities for production, our range, technical guidance, and fast dispatch help you find the exact fastener for the job. If you are unsure which size, grade, or finish suits your application, our team can help you match the right product to your load, environment, and standard, so your assemblies are reliable and built to last.

Frequently Asked Questions (FAQs)

1. What is a socket head bolt used for?

Socket head bolts are used wherever high clamping force and a compact head are required: machine tooling, jigs and fixtures, dies, hydraulic manifolds, robotics, and precision mechanical assemblies. The counterbored, flush-mount profile suits applications needing a clean surface.

2. What is the difference between a socket head bolt and a hex head bolt?

A socket head bolt uses an internal hex (allen) drive and has a compact cylindrical head, while a hex head bolt uses an external wrench. Socket heads offer higher torque capacity, cleaner recessed mounting, and closer spacing than hex heads.

3. What size allen key fits a socket head bolt?

Each metric size uses a standard hex key, for example M6 uses a 5mm key, M8 uses 6mm, M10 uses 8mm, and M12 uses 10mm. Always match the key to the socket to avoid rounding the recess.

4. What is Class 12.9 in socket head bolts?

Class 12.9 is a high-strength property class of alloy steel with roughly 1200 MPa tensile strength. It is the standard grade for socket head cap screws used in tooling and heavily loaded machinery.

5. Can socket head bolts be used in counterbored holes?

Yes  the cylindrical head is designed to sit inside a counterbore so the top sits flush with or below the surface, providing a clean, snag-free finish common in tooling plates and fixtures.

6. Are socket head bolts available in stainless steel?

Yes. We stock A2 and A4 stainless socket head cap screws for corrosion-resistant applications such as food machinery, marine equipment, and outdoor assemblies.

7. What is the difference between DIN 912 and ISO 4762?

They are equivalent standards for hexagon socket head cap screws with nearly identical dimensions. ISO 4762 is the international version; DIN 912 is the widely referenced German standard many engineers still specify.

8. What is a low head socket cap screw?

A low head socket cap screw (DIN 6912) has a reduced head height for applications with limited clearance. It uses a smaller hex key and carries a slightly lower torque rating than a standard socket head bolt.

9. What sizes do you stock for socket head bolts?

We stock M2 through M48 in coarse and fine thread, across alloy steel Class 12.9 and 10.9 and A2/A4 stainless, in a full range of lengths.

10. How do you prevent socket head bolts from loosening?

Use a thread-locking adhesive, spring or serrated lock washers, or correct torque to specification. The high clamp force of Class 12.9 bolts also improves vibration resistance in critical joints.