Expert Quality, a selection of imperial-sized high speed steel straight shank drills (with 5% cobalt) for specifically drilling out damaged threads prior to insertion of a Helicoil insert. The addition of cobalt to HSS adds strength and wear resistance for longer life. For use with UNC and UNF Helicoil® Thread Repair Kits. Display packed. DRAPER EXPERT 21/64" COBALT TWIST DRILL FOR HELICOIL® THREAD REPAIR KITS