Socket Head Cap Screw

What is a socket head cap screw?

A socket head cap screw has a round, cylindrical head with a hex socket in the top, tightened with a hex key (Allen key) or a hex socket bit. Hobson describes them as used where there is limited space for tightening from the outside, and commonly in machine assembly, especially for removable panels and parts. Because the tool goes inside the head, the head can sit in a counterbore or close to a wall where a spanner will not fit. People call them Allen bolts, Allen key bolts, cap screws or cap head screws.

Most of our metric range is class 12.9 alloy steel. Hobson notes that the 12.9 strength grade means fewer screws of the same size can achieve the same clamping force.

Which cap screw: alloy steel, zinc, stainless or nylon?

Pick the material for where the screw will live, then the size and length. For finish against exposure: plain and zinc plated for dry, indoor work; stainless outdoors. Hobson's corrosion guide accepts 304 or 316 stainless near the coast and in the sea spray zone, and calls for 316 within 2m of a pool.

• •Plain class 12.9 alloy steel (DIN 912): no coating; Hobson supplies plain fasteners lightly oiled to inhibit rust in the packet and help installation. For dry, indoor machine work.
• •Zinc plated (class 12.9, and class 8.8 in M6, M8, M12 and M16): a 5 micron zinc coating that Hobson recommends for indoor and non-corrosive conditions.
• •Zinc flake silver (class 12.9): a 10 micron coating that Hobson rates for high corrosion resistance without any risk of hydrogen embrittlement, thin enough to mate with standard internal threads.
• •304 (A2) and 316 (A4) stainless: for outdoor and damp work. 316 has better resistance near the sea, but neither 304 nor 316 is made to sit in seawater.
• •Neptune 316 (A4-80) and Bumax 88 (A4-80): 316 stainless in property class 80.
• •Bumax 109 (A4-100) and Bumax DX 129 duplex: Swedish-made high strength stainless, with full traceability (3.1 certificates for each item). Hobson lists Bumax 109 cap screws for oil and gas, marine and energy work, non-magnetic, for temperatures from -200 to +400 degrees C.
• •Natural nylon (DIN 912, M3 to M12): Hobson's nylon guide says nylon absorbs shocks and vibration, protects surfaces and gives good electrical insulation. Hobson publishes no strength or torque figures for the nylon cap screws.
• •Torx drive (304 stainless, ISO 14579, M3 to M12): a six-point star recess that Hobson says increases the torque you can apply, prevents cam out and helps the screw last.

Low head cap screws are reduced strength

Low head cap screws (DIN 7984) have a shorter head for tight spaces. Hobson's Soko guide explains that low head, button head and countersunk socket screws have reduced loadability because the stress area in the head is smaller than in the thread, so their head marking carries a leading 0: our steel low heads are class 010.9. Where the joint needs the full strength of the class, use a standard height cap screw. Hobson publishes no load ratings for joints made with these screws, so for structural or safety critical joints use the fastener your engineer specifies.

Metric socket head cap screw sizes and lengths

The everyday metric ranges, by material and size, smallest to largest. Lengths are measured under the head. Where a size comes in many lengths the table gives the range and the number of lengths; select a row to open that size in the selector and see every length.

Torx, high strength stainless, nylon, low head and fine pitch

Imperial cap screws: UNC, UNF and BSW

Imperial cap screws are sized by gauge (#2 to #10) or inch diameter, with a UNC (coarse), UNF (fine) or BSW thread. The alloy steel ones are to ASTM A574 (BSW to BS 2470), and the stainless ones to ANSI B18.3.

Socket head cap screw torque: class 12.9

Guide tightening torques from Hobson's class 12.9 socket head cap screw sheet, with the screw's tensile and proof loads from the same sheet. Hobson works the torques on a k factor of 0.2 for plain, burr-free threads with a light oil coating and 0.14 for zinc plated, for the first tightening of a single assembly. They are a guide only: they do not allow for the many things that change friction on a real job, and Hobson's engineers can advise a proper tensioning method for critical joints.

They apply to class 12.9 socket head cap screws in metric coarse thread, plain and zinc plated. They do not apply to the fine pitch 12.9 screws (M10 x 1.25, M12 x 1.5, M24 and M30 x 2), or to zinc flake, class 8.8, low head, stainless or nylon screws. The tensile and proof loads are properties of the screw, not the capacity of a joint.

Stainless socket head cap screw torque

Hobson's stainless socket head torque sheet works on a k factor of 0.2, which assumes burr-free threads and a good quality molybdenum disulphide (MoS2) lubricant. Hobson warns that stainless fasteners that are not lubricated or coated often seize and can have k factors over 0.35, so lubricate stainless threads. Use the column for the property class marked on the screw head: Neptune and Bumax 88 cap screws are class 80 (A4-80). For our standard metric cap screws, Hobson's stocklist gives 304 M6 to M20 as A2-70 and M24 as A2-50, and 316 M6 to M20 as A4-70 and M24 as A4-50. It gives no class for the smaller sizes, the Torx and low head screws or the imperial sizes, so check the head marking on those. The sheet does not cover Bumax 109 (A4-100), duplex or nylon.

UNC and UNF cap screw torque

From Hobson's Soko torque sheets for ASTM A574 socket head cap screws (1960 series), on the same k factors (0.2 plain, 0.14 zinc plated), for the first tightening of a single assembly. Figures are in lb-ft (multiply by 1.356 for Nm). The sheets have plain and zinc plated columns only, so they do not apply to the zinc flake screws, or to BSW, stainless or low head screws.

Fitting socket head cap screws

1. Use a hex key or bit that fits the socket fully; a worn key rounds out the socket.
2. Choose a length that gives full thread engagement in the nut or tapped hole.
3. Use a flat washer under the head on soft or slotted material.
4. Tighten to the torque for the class, finish and thread. The cap screw torques on this page do not apply to low head screws; for those, ask us or your engineer.
5. For stainless, lubricate the threads, or use a different stainless grade for the nut; Hobson's galling sheet says both help stop the threads seizing.

Socket head cap screws in Adelaide

Allmat supplies socket head cap screws, button heads, grub screws, nuts and washers to fabricators, mechanics, machine builders and trade across Adelaide and South Australia. Add them to a quote with the rest of your list, and we will price the lot.

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