Vermont American 20270 High Carbon Steel NC Fractional Bottoming Tap - 5/16-inch by 18 High carbon steel fractional bottoming taps are ideal for cutting right-hand internal threads Bottoming taps have 1 - 1/2 threads chamfered for tapping to the bottom of a hole Four flute design on all taps ensures fast and easy chip clearance. Required drill size is permanently etched on the tool for quick and easy reference.