Pilot hole

A pilot hole is either small hole drilled into a material to guide a larger drill to the appropriate location and ease the job of the larger drill, to allow for the insertion of another hole making tool, such as a knockout punch, that will produce the final size hole, or, in wood or plastic, to locate, guide, and provide clearance for a self threading screw to prevent damaging the material or breaking the screw.

Pilot for large holes

A pilot hole may be drilled the full extent of the final hole, or may only be a portion of the final depth. The pilot drill may be a standard twist drill, another type of drill bit appropriate for the material, or, when the primary purpose is precisely locating a hole, may be made with a short, stiff center drill.

The pilot hole also reduces the power needed to turn a large drill bit, and reduces the risk of breaking a large drill bit. For twist drill bits, the pilot size is usually selected to so that the chisel point of the larger drill does not need to remove any material, which reduces the chance of splitting the web of the bit. A pilot thus sized also prevents a larger drill bit from slipping on the material and guides the larger bit effectively. Very large holes may be stepped by drilling successively larger pilot holes before the final size drill is used.