Counterbore bungs are great as they allow for very clean mounting, as the head of a standard allen bolt will fit flush inside. These bungs are for 5/16" hardware. The outside measurements are 3/4" diameter and .625" long. These are nice when used in conjunction with our threaded 5/16"-18 bungs as they are the same diameter and make for a seamless mount. Our counterbore bungs are 1018 cold rolled steel for easy welding and are great for mounts of all sorts. Sold in 4 packs. Made in Ohio, USA