The Combo Drill Set 00131 includes: Six aircraft quality precision twist drill bits for the most popular rivet and machine screw diameters: 3/32" long length; 1/8" long, short, and stub lengths; 5/32" short length; and 3/16" short length. All TightFit threaded shank drill bits are black oxided to add lubricity, prevent corrosion while enhancing appearance. The drill bits comply with National Aerospace Standards. The Standard provides the following description, "A special short flute, heavy duty drill designed for drilling the hard, high strength alloys used in aircraft manufacturing." The drill bits are made from M2 or M7 tool steel with self-centering, 135 degree, P3, split points and a Rockwell C scale hardness of 63.5 to 66. They are manufactured as right-hand tools with 5/16" hex wrench flats and 1/4-28 UNF-2A threads. TightFit threaded shank drill bits are concentric, self-tightening, non-slipping, long life tools designed for work in limited space situations and work well in metal, wood or plastic.