Minimum Plain Bore

A MPB (Minimum Plain Bore) or Pilot Hole (as it is also referred to) is a 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 reamer or boring bar. MPB’s are not finished bores and therefore can vary in diameter. MPB’s do not have keyways or setscrews. 

Some smaller diameters use a center drill (a small V in the center of the part). The purpose of center drilling is to drill a hole that will act as a center of rotation for possible following operations. Center drilling is typically performed using a drill with a special shape, known as a center drill.