Master Mechanic, #21, 3-1/4, Black Oxide Wire Gauge Number Drill Bit, 135 Split Point Eliminates Walking & Provides Quick Starting, Surface Treated With Black Oxide Finish For Abrasion Resistance & Improved Chip Flow, Designed For Precision Hole Sizing Such As Tapping Threaded Holes, Wire Gauge Diameters Fall Between Fractional Sizes For Most Precise Hole Sizes, Taps Through Sizes 1/4-28 Require The Use Of Wire Gauge Bits, Carded.