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12.9 High Tensile Socket Cap Screws - Cap Head Bolts

12.9 High Tensile Socket Cap Screws - Cap Head Bolts

12.9 high tensile self colour socket cap screws, also called cap head bolts or Allen bolts, have a cylindrical head & an internal hex drive that's tightened with an Allen key. Made to the DIN 912 standard, also known as ISO 4762, 12.9 high tensile is chosen when clamping strength is important. The self colour finish is a deep near-black that comes from the heat treatment, not a coating. Stocking a range of metric sizes, & no minimum order.

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About 12.9 High Tensile Socket Cap Screws

What they are

12.9 socket cap screws have a cylindrical head with a recessed hex socket, driven by an Allen key. The 12.9 marking is the property class, and it is the highest one commonly used for socket cap screws, and are perfect wherever a joint has to be pulled up tight and stay tight. They are made from alloy steel that is quenched and tempered to reach that strength, and they follow DIN 912, the international equivalent of ISO 4762, the same standard used across our socket cap screws range.

Where they're used

The internal drive lets you tighten them in tight spaces, or sink them into a counterbored hole so the head sits flush or below the surface. 12.9 is ideal for use on machinery, engine and gearbox work, motorsport, tooling, jigs and fixtures, and anywhere a high clamping load is doing real work. If a joint has to hold under vibration or heavy load, this is the one.

The self colour finish

These come self colour, which simply means no coating has been added. On high tensile steel the heat treatment that gives 12.9 its strength also darkens the surface, so self colour here is a deep near-black rather than the bare grey you get on untreated steel. You will hear the same finish called black oxide, and the trade uses the two terms fairly interchangeably. The important part is that it is not weather protection. Self colour steel will rust if it gets wet and stays wet, so keep these dry or protected. If the job is outdoors or damp, A2 stainless is the sensible choice, and near salt or chemicals it is A4 stainless. Not sure which suits your job? Our grade guide runs through it.

Sizes

We stock 12.9 socket cap screws across a range of metric diameters, from M1.6 up to M10, in plenty of lengths. Shorter lengths are fully threaded and longer ones come partially threaded as standard. There is no minimum order, so you can buy the exact quantity you need. For the full sizing tables and a downloadable PDF, head to our cap head bolt dimensions guide.

12.9 High Tensile Socket Cap Screw FAQs

It is the property class, and it tells you how strong the fastener is. The higher the numbers, the higher the tensile strength, and 12.9 is the highest class in common use for socket cap screws. It is a strength marking rather than a material or a finish, so a 12.9 screw is chosen for load, not for looks or corrosion resistance.

Yes, high tensile is the everyday name for them. Any fastener heat treated to reach a high property class gets called high tensile, and in socket cap screws that usually means 12.9. If someone asks for high tensile cap head bolts, these are what they are after.

Self colour means nothing has been added to the surface, no plating and no coating. On high tensile steel the quenching and tempering that creates the strength also darkens the steel, so self colour 12.9 comes out a deep near-black instead of bare grey. You will see the same finish described as black oxide, and the two terms get used interchangeably in the trade. Either way it is an appearance, not a protective layer.

Yes, if they get wet and stay wet. Self colour and black oxide finishes are there for appearance and for the steel underneath, not for weather protection. Keep them dry, use them indoors or somewhere protected, or add your own protection. For outdoor work A2 stainless is the better answer, and for coastal or chemical exposure it is A4.

You can, but they will not last outdoors without help. If it is a sheltered spot, or the screw is buried inside an assembly and stays dry, they are fine. Left out in the weather they will start to rust. Where you need both strength and weather resistance, stainless is the trade off, since stainless is not as strong as 12.9 but it will not rust away on you.

Yes. Our 12.9 socket cap screws are made to DIN 912, the international equivalent of ISO 4762, the same standard used across the socket cap screws range.