3/8-24 tap drill size.
For a 3/8-24 tap, use a Q drill (0.3320 in, 8.43 mm). This gives about 79% thread engagement. For a clearance hole, use a 25/64 in drill (0.3906 in).
3/8-24 at a glance
| Item | Value |
|---|---|
| Thread | 3/8-24 (Unified UNF fine) |
| Major diameter | 0.3750 in (9.52 mm) |
| Threads per inch | 24 (pitch 0.0417 in, 1.058 mm) |
| Tap drill | Q drill: 0.3320 in (8.43 mm) |
| Thread engagement | about 79% |
| Clearance drill | 25/64 in drill: 0.3906 in (9.92 mm) |
How the tap drill is chosen
- Major diameter: 0.3750 in. Threads per inch: 24.
- For 75% thread: 0.3750 − 0.9743 ÷ 24 = 0.3344 in.
- Use the standard drill nearest that size: Q drill (0.3320 in), about 79% thread.
Drill the tap hole, deburr it, then tap with cutting fluid. In blind holes, drill deeper than the thread length: allow about 2–3 thread pitches below the last full thread.
- Other pitch: 3/8-16 tap drill size
Common questions
What size drill for a 3/8-24 tap?
For a 3/8-24 tap, use a Q drill (0.3320 in, 8.43 mm).
What is the clearance hole for 3/8-24?
Use a 25/64 in drill (0.3906 in, 9.92 mm). A clearance hole lets the screw pass through without threading.
What thread percentage does this drill give?
About 79%. Machinery's Handbook defines percent of thread as 76.98 × TPI × (major diameter − drill diameter). 75% is the common target: more engagement adds little strength but makes tapping much harder.
Can I use a slightly larger drill?
Yes, in hard materials such as stainless steel or titanium. A larger drill gives a lower thread percentage, which makes tapping easier and reduces tap breakage. Use the formula above to check the result.
Sources
- Tap drill sizes: Machinery's Handbook, 31st ed. — tap drill sizes for 75% thread.
- Drill diameters: ANSI B94.11M — twist drill sizes (number, letter and fractional series).
- Thread dimensions: ASME B1.1 — Unified inch screw threads.
Other tap drill sizes
#4-40 · #6-32 · #8-32 · #10-24 · #10-32 · 1/4-20 · 1/4-28 · 5/16-18 · 3/8-16 · 1/2-13
Open the full tap drill chart (UNC, UNF and metric, with PDF) →