Screw types compared: socket head, countersunk, set, carriage, tapping and self-drilling screws, with their DIN and ISO standards

How socket head, button head, countersunk, low-head and set screws differ, what DIN 912, ISO 7380, DIN 7991, DIN 916, DIN 603, DIN 965, DIN 7981 and DIN 7504 stand for, and which screw to order for machinery, timber and sheet metal.

Updated October 18, 2026

A screw is defined by its drive (hex socket, slot, cross recess or external hex), the shape of its head, and the way it holds: a machine screw has a metric thread and goes into a nut or a tapped hole, while a tapping screw or self-drilling screw forms its own thread in sheet metal. This guide compares the types we supply, the standards they are made to and the classes they come in. For what the property classes 8.8 and 10.9 mean in strength, see the bolt grades guide.

The screw types at a glance

Hex socket screws

The socket head cap screw to DIN 912 / ISO 4762 is the standard high-strength machine screw. Its cylindrical head is as tall as the thread diameter and fits a counterbore, so it sits flush in machine parts, jigs and tooling, and the hex socket takes a high tightening torque in a small space. We list it in carbon steel classes 4.8, 8.8 and 10.9 and in A2 and A4 stainless steel.

• Button head (ISO 7380): a low, rounded head for covers, guards and visible panels where a flat profile matters. The head and socket are smaller than DIN 912, so it is not a like-for-like replacement in highly loaded joints.
• Countersunk socket (DIN 7991 / ISO 10642): a 90° head that sits flush in a countersunk hole, used on plates, slides and fixtures where nothing may protrude.
• Low head (DIN 6912): a shorter cylindrical head for shallow counterbores and tight spaces, with a smaller drive than DIN 912.

Set screws

A set screw (grub screw) has no head: it is driven by a hex socket and fully sinks into a tapped hole to lock a collar, pulley or gear on a shaft. DIN 916 / ISO 4029 has a cup point that bites into the shaft. Set screws are specified by hardness class under ISO 898-5, not by the 8.8-style classes of bolts; the ones we list are class 45H. Other point shapes exist in neighbouring standards: flat point (DIN 913), cone point (DIN 914) and dog point (DIN 915).

Carriage bolts

A carriage bolt to DIN 603 / ISO 8677 has a smooth mushroom head and a square neck under it. The neck locks in timber or in a square hole, so the bolt cannot turn and is tightened from the nut side only. It is used for timber frames, gates, trailers and steel-to-timber connections, where a rounded head on the outside face is safer than a hex head.

Slotted and cross recess machine screws

Slotted and cross recess screws are light-duty machine screws for electrical enclosures, covers and small assemblies, usually in class 4.8. The slotted cheese head screw to DIN 84 / ISO 1207 has a cylindrical head with a straight slot. The cross recess countersunk screw to DIN 965 / ISO 7046 sits flush in a 90° countersink. A cross recess is either Phillips (H) or Pozidriv (Z); use the matching bit, since the two are not interchangeable under load.

Tapping and self-drilling screws for sheet metal

Tapping and self-drilling screws do not use a nut. Their thread is a coarse ST thread to ISO 1478, sized by diameter in millimetres (ST 4.2 is about 4.2 mm), not by M sizes.

• Tapping screw (DIN 7981 / ISO 7049): needs a pre-drilled pilot hole, then forms its thread in thin sheet. Pan head with cross recess, for sheet metal parts, ducts and enclosures.
• Self-drilling screw (DIN 7504 K / ISO 15480): a drill point makes the hole and the thread in one operation, so no pre-drilling is needed. The hex washer head spreads the load on cladding and sheet. The total thickness it can drill depends on the length of the drill point, so check it against the sheets being joined.

How to order

State the standard, the size and length, the class and the finish, for example DIN 912 M10×40 10.9 zinc plated or DIN 7504 K 4.8×25. Machine screws are sized by thread diameter and length under the head; countersunk screws are measured over the full length including the head. Carbon steel screws are quoted plain or zinc plated, and carriage bolts also hot-dip galvanized; for wet and outdoor conditions choose A2 or A4 stainless steel. Screws are sold per piece, and one request can cover nuts and washers in the same grade.