1/4" x 20, National Course Thread, Fractional Tap, Carbon Steel, Ideal For Routine Maintenance, Repair Work & For Rethreading Where High Accuracy & Tolerance Thread Is Not Critical, Straight Flutes Are Ground To Provide Rapid Chip Removal For Efficient Threading, Designed For Hand Threading Work In A Variety Of Materials Taps Produce A Class 2B Thread, Use Drill Bit #37 Or 3/16".