ASME B18.2.3.9M is the American Society of Mechanical Engineers' standard for Metric Heavy Hexagon Flange Bolts. Unlike standard flange bolts, the "Heavy" series features a larger wrenching height and an optimized flange geometry to withstand the extreme clamping forces required in structural and heavy machinery applications.

Primarily used in North American projects requiring metric specifications, this standard ensures that the fastener provides a high level of load distribution. The integrated flange eliminates the need for auxiliary washers, simplifying assembly in heavy-duty chassis, bridge components, and industrial pressure vessels where vibration resistance and high-tensile integrity are paramount.

The technical focus of ASME B18.2.3.9M is High-Tension Load Bearing. By utilizing a "Heavy" series head, the fastener allows for higher torque levels without the risk of rounding off the hex corners. The large bearing surface area of the flange ($D_c$) is engineered to maintain clamping force even when used with oversized or slotted holes, common in large-scale structural steelwork. This standard specifically addresses the transition radius between the shank and the flange to maximize fatigue life under dynamic loads.