1/2-13 tap drill size.
For a 1/2-13 tap, use a 27/64 in drill (0.4219 in, 10.72 mm). This gives about 78% thread engagement. For a clearance hole, use a 33/64 in drill (0.5156 in).
1/2-13 at a glance
| Item | Value |
|---|---|
| Thread | 1/2-13 (Unified UNC coarse) |
| Major diameter | 0.5000 in (12.70 mm) |
| Threads per inch | 13 (pitch 0.0769 in, 1.954 mm) |
| Tap drill | 27/64 in drill: 0.4219 in (10.72 mm) |
| Thread engagement | about 78% |
| Clearance drill | 33/64 in drill: 0.5156 in (13.10 mm) |
How the tap drill is chosen
- Major diameter: 0.5000 in. Threads per inch: 13.
- For 75% thread: 0.5000 − 0.9743 ÷ 13 = 0.4251 in.
- Use the standard drill nearest that size: 27/64 in drill (0.4219 in), about 78% 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 1/2-13 tap?
For a 1/2-13 tap, use a 27/64 in drill (0.4219 in, 10.72 mm).
What is the clearance hole for 1/2-13?
Use a 33/64 in drill (0.5156 in, 13.10 mm). A clearance hole lets the screw pass through without threading.
What thread percentage does this drill give?
About 78%. 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 · 3/8-24
Open the full tap drill chart (UNC, UNF and metric, with PDF) →