Socket Head Cap Screw
Bolts · SHCS · Allen bolt · socket screw · cap screw
Definition: A socket head cap screw has a cylindrical head with a hexagonal socket drive (Allen key). It offers high strength in a compact head profile.
Anatomy
■ Head■ Shank■ Threads■ Point
Governing standards: ASME B18.3 · ISO 4762 · DIN 912
Common Sizes
| Inch | Metric | Head width | Head height |
|---|---|---|---|
| #8-32 | M4 x 0.7 | 9/32 | 1/8 |
| 1/4-20 | M6 x 1.0 | 3/8 | 1/4 |
| 3/8-16 | M10 x 1.5 | 9/16 | 3/8 |
Representative sizes. Full dimensional data lives in the standards listed below.
Grades & Strength
| Grade | Material | Tensile (psi) | Marking |
|---|---|---|---|
| Alloy steel (black oxide) | alloy steel, Q&T | 180,000 | Grade 9 / ISO 12.9 equivalent |
| Stainless (A2/A4) | 304 / 316 stainless | 100,000 | A2-70 / A4-80 |
Torque Reference (in-lb)
| Diameter | Grade 2 | Grade 5 | Grade 8 |
|---|---|---|---|
| M4 (0.7) | 22 | 34 | 47 |
| M6 (1.0) | 74 | 112 | 158 |
| M10 (1.5) | 330 | 500 | 700 |
Dry, non-lubricated assembly values. Lubricated threads can require 20–30% less torque. Verify against your standard.
Materials
- Alloy steel (black oxide)
- Zinc-plated alloy steel
- Stainless steel 304 / 316
Pros & Cons
Pros
- Highest strength per diameter of common fasteners
- Compact head fits in recessed or tight spaces
- Clean, modern appearance
Cons
- Needs the exact size Allen key
- Hex socket can strip if the wrong key or angle is used
- More expensive than a hex bolt
Typical Uses
- Machinery and tooling
- Robotics and automation
- Bicycles and automotive
- Precision equipment and 3D-printer frames
Alternatives & Related
- Hex Bolt — Cheaper and easier to drive with a common socket
- Machine Screw — Smaller, fully threaded screws for tapped holes
Where to Buy
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FAQ
What is a socket head cap screw used for?
High-strength assembly where space is limited — machine tools, engines, bicycles, robotics, and any joint that must be tightened from above with an Allen key.
Socket head cap screw vs hex bolt?
The SHCS is stronger per size, has a smaller cylindrical head, and is driven by a hex key rather than an external wrench. The hex bolt is cheaper and tolerates dirty or damaged conditions better.