DIN 6921 is the foundational German standard for Hexagon Bolts with Flange. These fasteners are characterized by a circular "flange" under the hex head, which acts as a built-in washer to distribute the clamping load over a larger surface area while maintaining a low profile.

In modern engineering, DIN 6921 bolts are preferred over standard hex bolts (like DIN 931) in automotive frames, engine mounts, and structural steelwork. The integrated flange eliminates the need for separate washers, reducing assembly time, minimizing parts inventory, and providing superior resistance to vibration-induced loosening.

The technical focus of DIN 6921 is Surface Pressure Optimization. By increasing the bearing area ($d_c$), the flange significantly reduces the contact pressure on the clamped part, preventing the fastener from "digging in" or damaging soft metal surfaces. While standard DIN 6921 features a smooth bearing surface, serrated versions (often referred to as DIN 6921 Serrated) are used where maximum locking grip is required to bite into the mating surface.