Fastener Head Types

What the head has to do

A fastener head does at least one of four jobs: it transfers installation torque, provides a bearing surface that spreads clamp load, sits flush with or below the mating surface, or gives a hand something to grip. Most head designs are built around one of those jobs, and the choice should follow from the job, not the other way around.

Torque transfer is the most visible role: a hex head exists because a wrench needs six flats, and a socket cap head because an internal hex lets the tool engage from above. Bearing area is quieter but often more important — the underside of the head presses the joint together, and on soft material a small head will pull through or leave a ring; hence flange, truss and washer-face heads. Flush mounting changes the geometry, because a flat or countersunk head must match the countersink angle in the part. Hand grip is the last job: wing, thumb and knurled heads suit a fastener adjusted by fingers. The table below lists the sixteen head styles we produce.

Wrenching heads versus recessed drives

One early decision splits heads into two families: heads the tool grips from outside, and heads the tool engages from inside. A hex head is the classic external-drive head — a wrench wraps around it, so it survives repeated assembly and high torque, carrying more torque than a recess because the load spreads over the full perimeter of the flats.

A recessed head turns that inside out: a socket cap, button or flat head carries the drive in the middle, so the head can be smaller, rounder and recessed into a close counterbore. The trade-off is engagement — the recess is smaller than an external hex, so it is more sensitive to wear, to an undersized bit and to an off-square driver. Where torque is high or the fastener is removed many times, external hex usually wins; where the head must be compact, an internal drive is the answer. The companion page on drive types covers the recesses themselves.

Heads that sit flush

Flush heads are chosen when the fastener must not stand proud — for a mating face, a moving part passing over it, or a clean exterior. The flat head is the workhorse: it seats into a countersink cut at a standard included angle (82° and 90° are the common ones), and the head angle must match or the head contacts only on a thin line and loosens.

Countersunk socket heads bring the same behaviour to an internal-drive screw. What matters is the included angle and the head diameter: get the angle wrong and the joint never seats, get the diameter wrong and the countersink leaves a gap or swallows the head. Because head height controls flushness, it is usually held tighter than a plain hex head — a feature we hold at ±0.01 mm when the drawing calls for it. When the countersink depth is critical, our drawing requirements page describes what to put on the drawing.

Heads that spread load

Where the base material is soft or the clamp force is high, the underside of the head becomes the load path. A standard hex head with a washer does the job, but the washer is a second part that can be lost or misassembled. Flange heads build the washer into the heading or forging. Truss heads go further — a low, wide dome spreads pressure over the largest area of any common machine-screw head, which is why they suit thin sheet and soft material where the screw must not pull through. Washer-face heads on hex cap screws give a smooth, flat bearing ring that reduces surface damage.

For soft aluminium, brass or copper parts, bearing area matters as much as the thread. A small head tightened hard can indent or yield the mating face, and a flange, truss or washer-face head is often cheaper than a larger thread.

Heads we machine to drawing rather than to a standard

Most of what we ship in volume is a standard head, cold-headed or forged and then finished. But a large share of the enquiries that reach a machinist arrive precisely because the head is not in a catalogue. A buyer who needs a hex head 0.5 mm smaller than standard, a flange diameter that must clear a boss, a carriage square neck for a specific slot, a wing head in an unusual span, or a weld head with a particular projection has nowhere to buy it off the shelf. That is the usual reason to come to us rather than to a distributor.

Some of these heads resist rotation in the workpiece: carriage and square-neck heads dig a square shoulder into timber or a mating slot so the bolt cannot spin while the nut is tightened. Hand-adjustment heads — wing, thumb and knurled — are machined or headed where the operator must tighten or release without a tool. Weld heads and weld studs carry a projection or pilot that the welder fuses to a plate, so the head shape is set by the joint.

Choosing a head for your application

Start from the job, then pick the head. Driven many times at high torque, stay with external hex. Needing to sit flush, decide the countersink angle and head diameter first. Clamping soft material, put bearing area on the underside. With one side accessible, a carriage or square-neck head stops the spin. Adjusted by hand, choose wing, thumb or knurled.

Then check the table below against those requirements before you commit to a drawing. When a standard head fits, we supply it from our material range (aluminium, stainless, carbon and alloy steel, brass, copper, titanium). When it does not, send the drawing through request a quote and we will quote within 24 hours with DFM feedback, including whether the head should be headed, forged or machined. See also custom bolts and custom screws. Samples from one piece run 5–10 days; production from 50 pieces runs 25–35 days after approval, with a material certificate (MTR) and pre-shipment photos with the order.