Drill America Counterbore, Aircraft Series, 1" Size, 3/16" Pilot Hole, 1/4" Shank, 3-1/8" OAL

Counterboring is the enlargement of a portion of a hole that has previously been formed. If the counterbored portion is shallow (1/8"" or less), the operations is usually known as spot-facing; for example, a flat seat for a bolt head or nut. If the hole is enlarged to a considerable depth, it is designated as counterboring.