Why the Head Shape Matters
The head of a bolt is its toolbox: it determines how you drive it, how much torque it takes, whether it sits flush, and whether it resists tampering. Choosing the right head type is as important as choosing the strength grade — a socket head cap screw and a hex bolt of the same size are not interchangeable in most assemblies.
This guide covers the six head types you will meet most often in metric hardware, with the DIN standard that defines each one.
The Six Common Head Types
Hex Head (DIN 933 / DIN 931)
The universal default. Six flats for a wrench, large bearing surface, highest torque capacity with standard tools, available in every strength class and finish (M6–M48, classes 4.8–12.9). Full detail in our DIN 933 dimension chart.
Socket Head (DIN 912)
Cylindrical head with an internal hex drive, sits in a counterbore, typically class 12.9. The machinery standard for compact, flush joints. Compare it head-to-head with hex bolts in our socket screw guide — or see the DIN 912 class 12.9 product line.
Flange Head (DIN 6921)
A hex head with an integrated washer-face flange. The flange spreads the load and — in the serrated version — bites the surface for vibration resistance, eliminating separate washers and lock nuts. Very popular in automotive and machinery assembly.
Square Head (DIN 479)
Four flats give a large wrench grip and a natural anti-rotation feature in wood and retro-fit work — the square head can be held with a wrench from the side or secured against turning in a square recess. Mostly used in machinery, waterworks and restoration. See square head bolts DIN 479.
T-Head (DIN 261)
The head forms a “T” that slides into a slot or channel, letting the bolt travel along the groove — the foundation of clamping beds, machine tables and structural slot systems. See T-head / T-slot bolts DIN 261.
Dome Head with Square Neck (DIN 603)
The carriage bolt: smooth domed head for a clean, tamper-resistant finish side, square neck that locks into wood. Covered in depth in our hex vs carriage vs lag screw comparison.
Comparison Table
| Head Type | Standard | Drive | Sits Flush? | Typical Use |
|---|---|---|---|---|
| Hex | DIN 933/931 | Wrench / socket | No | Structural, general fabrication |
| Socket cap | DIN 912 | Hex key | Yes (counterbore) | Machinery, fixtures, tooling |
| Flange | DIN 6921 | Wrench / socket | No (flange face) | Automotive, vibrating machinery |
| Square | DIN 479 | Wrench | No | Machinery, retrofits, waterworks |
| T-head | DIN 261 | Wrench (nut side) | Into slot | Clamping beds, slot systems |
| Carriage | DIN 603 | Nut only | Yes (dome) | Wood construction, clean finishes |
Choosing by Constraint
- Maximum torque, simplest tools: hex head.
- Flush surface or tight side clearance: socket cap screw.
- Vibration + speed of assembly: serrated flange bolt — skips the washer step.
- Adjustable position along a slot: T-head bolt.
- Head must not turn during tightening: square head or carriage bolt (square neck).
- Special geometry or non-standard heads: custom CNC fastener production from drawings.
And don’t forget the load path: whichever head you choose, pair it with the right washer and matching nut, and torque it per our metric torque chart.
Frequently Asked Questions
Which head type is strongest?
Head shape rarely determines joint strength — the property class does. A class 12.9 socket screw out-clamps an 8.8 hex bolt, but a 12.9 hex bolt out-clamps it right back. Match the head to the assembly geometry, and the grade to the load.
Why not just use socket screws everywhere?
Cost and access. Socket screws need counterboring (machining time), hex keys deliver less torque than wrenches, and 12.9 fasteners are restricted in finishes. Hex bolts win on economy and torque in open joints.
Are flange bolts a substitute for hex bolt + washer?
In most cases, yes — that is exactly what they were designed for. Exceptions: very soft or oversized holes (use a large flat washer), and slip-critical structural joints where the project spec mandates hardened washers.
Can T-head bolts be used in structural applications?
Yes — DIN 261 T-bolts anchor into structural channels (e.g., DIN 50 profiles) for adjustable steel connections; our T-head bolts page lists stocked sizes.