Definition - What does Pilot Bore mean?
A pilot bore is the initial hole drilled during any drilling sequence and spans the entire length of the hole. After the pilot bore, the result normally needs additional processing in the form of back reaming or other enlargement methods. The pilot bore is usually performed with a steerable head, and is used during directional drilling operations and 2-pass microtunneling systems.
Trenchlesspedia explains Pilot Bore
When the pilot bore is complete, the bit emerges in the exit pit and the drill bit is removed. A back reamer is then attached to enlarge the hole and provide a structurally sound and smooth bore for the installation of pipe. The finished hole may be as much as 50 percent bigger than the product pipe. In some instances, the back reaming pass may be skipped, instead, the predetermined product pipe will be attached to the drill string and pulled back through the bore.