Hex / Allen Drive (Internal Hexagon)
Direct Answer
The Hex drive (Allen socket / Internal Hexagon, standardized under ISO 4026 and DIN 912) is a 6-sided symmetrical polygonal drive recess developed in the early 1900s by William G. Allen. Widely used in socket head cap screws, bicycle components, and flat-pack furniture, it provides high clamping torque with compact cylindrical head profiles.
Source: ISO 4026 / ISO 4762 / DIN 912 / ASME B18.3
Specs
| Standard Specifications | ISO 4762 / DIN 912 (Socket head cap screws) / ASME B18.3 |
| Drive Geometry | Symmetrical 6-sided internal hexagon socket |
| Metric Standard Sizes | 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0, 12.0, 14.0, 17.0 mm |
| Imperial Standard Sizes | 0.050", 1/16", 5/64", 3/32", 1/8", 5/32", 3/16", 1/4", 5/16", 3/8" |
| Cam-Out Tendency | Low (Requires axial insertion depth; strips if bit is worn or undersized) |
| Primary Applications | Machine tools, bicycle components (4/5/6mm), 3D printers (2.5/3mm), flat-pack furniture |
Internal Socket Mechanics
The internal hex drive allows the screw head to remain completely cylindrical (such as DIN 912 socket head cap screws), allowing the fastener to be recessed into counterbored holes flush with the surface of machine castings without needing clearance for an external box wrench.
Metric vs Imperial Allen Key Confusion
Because metric and imperial hex keys are visually indistinguishable, using an imperial key in a metric socket (e.g. a 5/32" [3.97 mm] key in a 4.0 mm metric socket) creates 0.03 mm of play. Under high torque, the sharp corners of the key round off the internal hexagonal flats, permanently stripping the socket.