This guide block is designed for use with the insert installation Tap and Drill bit sold on this site.  The block has a drill bushing for an "F" size drill (.257") and a tapping guide bushing for the 5/16-18 tap.  Use it to aid in installing the threaded inserts perfectly straight.  It is also compatible with the 3in1 installation tool, which can be used to align the guide block when drilling out old holes.