An adjustable apparatus for guiding medical instruments used in attaching bone plates. The apparatus has a handle assembly, guide member, a spacer, and an actuation member. The actuation member extends from the handle assembly and actuates the spacer between a position adjacent the guide member and a position spaced from the guide member. The spacer has a through hole that may axially align with a lumen extending through the guide member when in the first position and which is moved out of axial alignment with the lumen in the second position. The spacer may function as a depth stop to control the maximum depth a drill bit can extend from the guide member.