Solid steel construction. D and C cell drilling guide for drilling center hole. Uses a 3/16 inch drill bit and I recommend a few drops of oil to keep the hole from wearing out over time. Drills a hole dead center in the plug. Made from 4130 cold rolled solid bar stock. Don't skimp on a center tool and go with the best. There are cheaper ones available, you get what you pay for. The D size end fits Dorman 555-104B or Sealed Power 381-3179 or any brand with an ID of 1.23 including the new J&S Tactical Stainless Steel plugs. The C size end fits Dorman 555-017B or Sealed Power 381-3016 or any brand with an ID of .88 This tool does NOT form the plug, it only drills a center hole. If you want to form it as well, check my other listings.