Generally, countersunk-head fasteners are used in applications where the connected surface must remain smooth and free of protrusions, as the head can be seated flush within the component. Round-head fasteners can also be seated into the component material. Square-head fasteners allow for greater tightening torque, though they are significantly larger in size. Furthermore, to ensure secure locking after installation, some bolts feature holes in the head or shank; these holes allow for the insertion of locking pins or wires to prevent the bolt from loosening under vibration.
In some bolts, the unthreaded portion of the shank is intentionally reduced in diameter; these are known as waisted bolts. Such bolts are particularly advantageous for connections subject to variable or dynamic loads. Steel structures utilize specialized high-strength bolts, which typically feature enlarged heads and distinct dimensional specifications. Additionally, there are bolts designed for specific applications: T-slot bolts-most commonly found in machine tool fixtures-feature a unique shape with the sides of the head cut away; anchor bolts are used to secure machinery to the ground and come in a wide variety of shapes; and U-bolts, as previously mentioned, serve specific clamping purposes. There are also specialized weld studs, which feature threads at one end and a plain shank at the other; the plain end can be welded directly onto a component, allowing a nut to be fastened directly to the threaded end.