What type of hole allows the head of a bolt or screw to rest on a flat surface below the outer surface of the part?

The type of hole that allows the head of a bolt or screw to rest on a flat surface below the outer surface of the part is known as a counterbored hole. This hole is specifically designed with two different diameters: the larger diameter accommodates the head of the bolt or screw, while the smaller diameter allows the shank to pass through to the deeper portion of the hole. The counterbore creates a flat-bottomed recess that ensures the fastener sits flush with or below the surface of the material, providing a clean and smooth finish, which is essential in applications where protruding fasteners could be problematic.

In contrast, a recessed hole typically refers to shapes designed to hold a specialized screw that has a particular head shape, such as a Phillips or hex, but does not specifically imply that the head is below the outer surface. A pilot hole is generally used as a guide for drilling a larger hole or is the starting point for a screw, not designed to accommodate the entire head below the surface. Meanwhile, a slotted hole is designed to allow for adjustment or movement of a fastener but does not provide the same function as a counterbored hole in terms of depth and flushness.