Hex Head (Hexagon Bolt)
Direct Answer
Hex Head Bolts (standardized under ISO 4014 / DIN 931 for partially threaded bolts and ISO 4017 / DIN 933 for fully threaded tap bolts) feature a 6-sided external polygon head driven by open-end wrenches, box wrenches, or ratcheting sockets. It is the global benchmark for high-torque heavy structural and automotive clamping.
Source: ISO 4014 / ISO 4017 / DIN 931 / DIN 933 / ASME B18.2.1
Specs
| Standard Specifications | ISO 4014 (Partial thread) / ISO 4017 (Full thread) / DIN 931 / ASME B18.2.1 |
| Head Geometry | 6-sided external hexagon with flat top face and circular washer face underside |
| Common Metric Wrench Sizes | M6 (10mm), M8 (13mm), M10 (16/17mm), M12 (18/19mm), M16 (24mm) |
| Common Imperial Wrench Sizes | 1/4" (7/16"), 3/8" (9/16"), 1/2" (3/4"), 3/4" (1-1/8") |
| Strength Markings (Metric) | Class 8.8, 10.9, 12.9 stamped directly on head top |
| Strength Markings (Imperial) | Grade 2 (plain), Grade 5 (3 radial lines), Grade 8 (6 radial lines) |
| Flanged Variant | DIN 6921 (Hex head with integrated wide washer flange) |
External Drive and Massive Torque Transfer
Because wrench sockets grip the entire exterior perimeter of the 6-sided head rather than driving an internal recess, hex bolts can sustain massive torque loads without stripping. In high-tensile automotive suspension and steel bridge construction, pneumatic impact wrenches apply hundreds of foot-pounds of torque directly to hex bolt heads.
ISO vs DIN Across-Flats (AF) Sizing Differences
In the 1980s, ISO updated wrench sizes for three standard metric bolt sizes to optimize steel material usage:
- M10: DIN uses 17 mm wrench $\rightarrow$ ISO specifies 16 mm.
- M12: DIN uses 19 mm wrench $\rightarrow$ ISO specifies 18 mm.
- M14: DIN uses 22 mm wrench $\rightarrow$ ISO specifies 21 mm.
Modern automotive mechanics maintain both DIN and ISO socket sets to service European and Asian vehicles.