5/16" counterbore to fit Bolt-type Rivet, Hidden-Bolt Rivet, and Blind Bolt 5/16" Head. The Counterbore, with interchangeable pilot, is a two part tool for countersinking holes in handle slabs and tang. It affords much flexibility for those who use a variety of rivet types. Select shank size the same diameter as the rivet head and pilot size the same diameter as the center section of the rivet. The pilots are held in place with an Allen screw. For your information the rivet type is listed with appropriate counterbore and pilot size for easy selection.