5/16″ x18 NC, Carbon Steel, National Course Thread Fractional Tap Plug, Ideal For Routine Maintenance, Repair Work And For Rethreading Where High Accuracy And 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, Drill Bit Letter F or 1/4″.