#8-32 tap drill size.
For a #8-32 tap, use a #29 drill (0.1360 in, 3.45 mm). This gives about 69% thread engagement. For a clearance hole, use a #18 drill (0.1695 in).
#8-32 at a glance
| Item | Value |
|---|---|
| Thread | #8-32 (Unified UNC coarse) |
| Major diameter | 0.1640 in (4.17 mm) |
| Threads per inch | 32 (pitch 0.0312 in, 0.794 mm) |
| Tap drill | #29 drill: 0.1360 in (3.45 mm) |
| Thread engagement | about 69% |
| Clearance drill | #18 drill: 0.1695 in (4.31 mm) |
How the tap drill is chosen
- Major diameter: 0.1640 in. Threads per inch: 32.
- For 75% thread: 0.1640 − 0.9743 ÷ 32 = 0.1336 in.
- Use the standard drill nearest that size: #29 drill (0.1360 in), about 69% 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.
Common questions
What size drill for a #8-32 tap?
For a #8-32 tap, use a #29 drill (0.1360 in, 3.45 mm).
What is the clearance hole for #8-32?
Use a #18 drill (0.1695 in, 4.31 mm). A clearance hole lets the screw pass through without threading.
What thread percentage does this drill give?
About 69%. 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 · #10-24 · #10-32 · 1/4-20 · 1/4-28 · 5/16-18 · 3/8-16 · 3/8-24 · 1/2-13
Open the full tap drill chart (UNC, UNF and metric, with PDF) →