2-1/2-4 tap drill size

    Use a 2-1/4 drill (2.2500 in) for a 2-1/2-4 cutting tap at 75% thread.

    Drills for 2-1/2-4 (UNC)
    Tap typeDrillDecimal (in)
    Cutting tap, 75% thread2-1/42.2500
    Cutting tap, 50% thread (hard material)2-11/322.3438
    Roll / form tap, 65% thread2-3/82.3750
    Pitch
    6.35 mm (4 TPI)
    Major Ø
    2.5000
    Pitch Ø
    2.3376
    Minor Ø
    2.2294

    2-1/2-4 tapping speeds and feeds

    Feed = pitch (0.2500 in per rev). Starting points for a general-purpose HSS cut tap:

    • Aluminum 6061 / 7075: 110 RPM · 27.5 IPM
    • Free-machining / mild steel: 30 RPM · 7.50 IPM
    • 304 / 316 stainless: 20 RPM · 5.00 IPM

    2-1/2-4 tap speeds and feeds: all materials, form taps and G84 code

    Open the tap drill calculatorRoll tap chartHelicoil chart

    Common questions

    What size drill for a 2-1/2-4 tap?
    Use a 2-1/4 drill (2.2500 in) for a 2-1/2-4 cutting tap at 75% thread.
    What drill for a 2-1/2-4 roll (form) tap?
    About 2.3895 in; the nearest standard drill is 2-3/8. Check your tap maker's table.

    Other UNC sizes

    Drill sizes from Machinery's Handbook and ASME B1.1/B1.13M; roll tap sizes from the form-tap formula. Tap and insert makers' tables are authoritative.

    Free for machinists & programmers

    Put your skills on a free talent profile

    Machines, controls, certs and saved calculations in one profile shops can find.

    Create free profile
    For your shop

    Bring shift handoffs to your shop

    Stop losing job status between shifts. Every station's notes, counts and issues, handed off cleanly.

    See shift handoffs