Dacromet coated hexagonal cylindrical head screw: A small bolt determines the life of large-scale equipment, anti-corrosion and corrosion-resistant, and there is no hidden danger of hydrogen embrittlement.

2026-11-04

1. Introduction: The Bolt That Holds Machines Together

Walk through any machine shop, robot cell, or wind-turbine nacelle and you will find the same fastener: a cylindrical-head screw with a hexagonal recess, tightened with an Allen key. This is the hexagon socket head cap screw, standardized as DIN 912 and ISO 4762. It is the default bolt for die molds, machine tools, hydraulic manifolds, and precision assemblies, because it concentrates high clamp load in a compact envelope and can be recessed into a counterbore.

2. Product Definition: Hexagon Socket Head Cap Screw

The geometry of this bolt is optimized for preload, not for flush appearance:

Cylindrical headwith a flat bearing face that distributes clamp load over a full circular area.
Internal hexagon socket(metric across-flats from 2.5 mm to 36 mm), transmitting torque through six faces with minimal cam-out.
Full-form metric thread(coarse or fine pitch per ISO 261), rolled rather than cut for continuous grain flow and better fatigue life.
Property classes 8.8, 10.9, and 12.9, covering tensile strengths from 800 to 1,220 MPa.

Thread	Head Dia. (dk)	Head Height (k)	Hex Socket (s)	Class 8.8 Tensile	Class 12.9 Tensile	Common Lengths
M5	8.5 mm	5.0 mm	4 mm	800 MPa	1,220 MPa	10–60 mm
M6	10.0 mm	6.0 mm	5 mm	800 MPa	1,220 MPa	12–80 mm
M8	13.0 mm	8.0 mm	6 mm	800 MPa	1,220 MPa	16–100 mm
M10	16.0 mm	10.0 mm	8 mm	800 MPa	1,220 MPa	20–120 mm
M12	18.0 mm	12.0 mm	10 mm	800 MPa	1,220 MPa	25–150 mm
M16	24.0 mm	16.0 mm	14 mm	800 MPa	1,220 MPa	30–200 mm

The head-to-thread ratio matters for corrosion design: a large cylindrical head gives the coating a broad, low-stress surface to bond to, while the deep socket recess is the most coating-critical zone of the entire screw — it is where rust first attacks and where a thin or incomplete film causes the bolt to seize.

Dacromet coated hexagonal cylindrical head screw

3. Installation: Torque, Preload, and Coating Interaction

A corrosion-resistant bolt is worthless if the joint loosens. Table 2 gives assembly torque for class 8.8 and 10.9 socket head cap screws with Dacromet coating (μ ≈ 0.14).

Thread	Class 8.8 Torque	Class 8.8 Preload	Class 10.9 Torque	Class 10.9 Preload
M5	6.5 N·m	4.0 kN	9.0 N·m	5.6 kN
M6	11.5 N·m	5.7 kN	16.0 N·m	8.0 kN
M8	28.0 N·m	10.4 kN	39.0 N·m	14.6 kN
M10	55.0 N·m	16.5 kN	78.0 N·m	23.1 kN
M12	96.0 N·m	24.0 kN	135 N·m	33.6 kN
M16	240 N·m	45.0 kN	335 N·m	63.0 kN

Use the correct key.A worn or undersized Allen key damages the socket and exposes bare steel — the exact place where corrosion starts.
Respect the counterbore.The head seats on its full bearing face; a chamfered or damaged counterbore reduces contact area and can crack the coating film under load.
Do not mix coating systemson the same joint without re-checking torque. A Dacromet screw into a plain nut has different friction than a coated screw into a coated nut.
Re-torque after settlementon soft gaskets or plastic housings; the dry film does not creep, but the clamped material may.

4. Conclusion

The Dacromet-coated hexagon socket head cap screw is a small component that decides the service life of a large assembly. Its cylindrical head and internal hexagon deliver the clamp load modern machinery demands; its zinc-aluminum flake coating delivers the anti-corrosion performance that outdoor, marine, and transportation service demands — without hydrogen embrittlement, without thread re-tapping, and without the galling penalty of stainless steel. Specify the build correctly, torque it correctly, and the bolt will outlast the paint on the machine it holds together.

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