Hex head bolts have hexagonal heads for firm wrench tightening. Also called engineer's bolts, they securely fasten infrastructure that requires high-tensile strength. These bolts are frequently used in confined spaces because wrenches can access the hex heads from all angles. Standard hex head cap screws allow for greater torque than circular screws. Structural and heavy hex bolts have larger, thicker heads than regular hex bolts . Flange bolts have a circular edge under the head that acts like a washer to distribute loads. Sealing hex head cap screws have a silicone O-ring under the head to prevent leaks and seal out moisture. Sanitary hex head cap screws have an FDA-compliant silicone O-ring seal under the head. Coil bolts screw into coil ties and provide clamping force to hold concrete forms together. Hex head stop bolts locate tapped workpieces in machining and finishing processes.