How to Choose Socket Head Cap Screws: DIN 912 / ISO 4762, Grade 12.9 and Variants

DIN 912 and ISO 4762 describe the same screw for practical purposes: a cylindrical head with a hexagon socket, a head diameter of about 1.5 times the thread diameter and a head height equal to the thread diameter. ISO 4762 replaced DIN 912, and the two are interchangeable for almost all sizes. ASME B18.3 is the inch-series standard for the same type of screw, with UNC or UNF threads, and is used in American-made equipment.

Do not mix inch and metric screws or keys. A 1/4 inch screw and an M6 screw are close in size, but the thread is different, and so is the key: an M6 screw uses a 5 mm key and a 1/4 inch screw uses a 3/16 inch key.

The default socket head cap screw is made from alloy steel to property class 12.9, with a minimum tensile strength of 1200 MPa and a hardness of roughly 39 to 44 HRC. That strength is why the head can be so small. Class 8.8 is also available (compare the classes in our hex bolts guide) and is used where 12.9 is more than the design needs, or where a softer screw is wanted. Stainless screws in A2-70 or A4-70 (see our stainless steel guide) are used for corrosion resistance, at a much lower strength of 700 MPa.

Do not replace a 12.9 screw with a stainless one in a loaded joint. A2-70 has roughly 60 percent of the tensile strength, and it will also stretch and lose preload sooner. Equally, do not tighten a 12.9 screw in a soft aluminium thread to its full torque: the thread will strip before the screw is stressed.

• 12.9 alloy steel: machine tools, dies, moulds, engines, hydraulic and precision equipment.
• 8.8: general machinery and frames where 12.9 is not needed.
• A2-70 / A4-70: food, medical, marine and outdoor equipment with modest loads.

The standard cap head (ISO 4762) needs a counterbore as deep as the head height. Where there is less depth, the low head version (DIN 7984) has a shorter head, but it carries less load and has a shallower socket. The button head (ISO 7380) has a domed, low profile that suits covers and guards, and the flat or countersunk head (ISO 10642 / DIN 7991) sits flush in a conical recess.

Each variant is driven by a different key size at the same thread diameter, so a toolkit set for cap screws may not fit the button head screw of the same thread. Tell your supplier the head type and key size when you order replacements.

The usual key sizes for ISO 4762 are 3 mm for M4, 4 mm for M5, 5 mm for M6, 6 mm for M8, 8 mm for M10, 10 mm for M12 and 14 mm for M16. Using a worn or wrongly sized key, or a key at an angle, rounds the socket and makes the screw impossible to remove. Good screws have a socket depth of about half the thread diameter or more, and a clean, sharp socket with no flash from forging.

A poor-quality screw often fails at the socket before it fails in the thread. Ask for the socket depth and the key size to be checked on a sample, and use a ball-end key only for pulling the screw in place, not for the final tightening.

Black oxide is the usual finish for class 12.9 socket screws. It is lightly oiled and gives mild protection indoors. For better corrosion resistance, a zinc flake coating (Geomet or Dacromet) is the preferred choice. Electro-zinc plating on 12.9 screws needs care: hard steel plated by an electrolytic process can absorb hydrogen and later fail suddenly under load, so the screws must be baked straight after plating, or a different coating chosen.

If your screws are used in a load-bearing, safety-related or fatigue-loaded joint, ask the supplier in writing how the finish is controlled. A supplier who cannot answer should not supply class 12.9 plated screws.

Check head diameter, head height, socket depth and key fit with a gauge key. Check the thread with go and no-go gauges. Look at the head: the manufacturer's mark and the number 12.9 should be stamped on the head for sizes of M5 and above. Ask for hardness results; a 12.9 screw measured below the specified hardness range is not a 12.9 screw, whatever the marking says.

For critical use, request a tensile or proof load test on samples and an EN 10204 3.1 certificate that links to your lot (how to read a mill test certificate).

Global Fasteners Exports is a trading and export company: our socket head cap screws are made by partner factories and we handle inspection, packing, documents and shipping. Because we buy in volume from those factories, prices are typically up to 30% lower than the market price.

For socket head cap screws, tell us the head type, the grade and whether the screws are plated, so the finish is matched to a hydrogen-controlled process.

Send the standard, thread size and length, grade or material, finish, and head type with the key size, and we confirm price, MOQ and lead time.

• Samples or detailed photos can be arranged before a bulk order; sample and courier costs are normally adjusted against the first order.
• Standard payment terms are 50% advance to confirm the order and the balance before dispatch.
• Documents: commercial invoice, packing list, certificate of origin and bill of lading, plus mill test certificates and inspection reports on request.
• Different products can be combined in one consolidated FCL or LCL shipment.