#4-40 UNC tap drill size
Tap #4-40 UNC with a #43 drill (0.0890 in, 2.26 mm), about 71% thread. Taps this fine break easily, so hand-tap with cutting oil and back off often.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #45 | 0.0820 | 2.08 | 92.4% | Tight: high tapping torque, tap breakage risk |
| #44 | 0.0860 | 2.18 | 80.1% | General purpose |
| 2.25 mm (metric drill) | 0.0886 | 2.25 | 72.1% | General purpose |
| #43 (chart drill) | 0.0890 | 2.26 | 70.8% | General purpose |
| #42 | 0.0935 | 2.37 | 57.0% | Low engagement: check strength |
| 3/32" | 0.0938 | 2.38 | 56.2% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #2-56 UNC | #50 (0.0700 in) | 1.78 mm | #43 / #41 |
| #3-48 UNC | #47 (0.0785 in) | 1.99 mm | #37 / #35 |
| #4-40 UNC (this page) | #43 (0.0890 in) | 2.26 mm | #32 / #30 |
| #5-40 UNC | #38 (0.1015 in) | 2.58 mm | #30 / #29 |
| #6-32 UNC | #36 (0.1065 in) | 2.71 mm | #27 / #25 |
Practical notes
At 40 TPI the basic thread depth is only about 0.0135 in per side, so small errors in drill size change engagement a lot. Use a sharp, correctly sized drill, a tap guide to keep the tap square, and limit thread depth to about 1.5 × diameter.
Thread-forming taps are popular for #4 sizes in aluminium and brass because they make no chips; they need a larger hole, roughly 0.0995 in (2.53 mm, D − 0.5 × pitch). Confirm against the tap maker's chart.
If a #4 thread in plastic or a printed part must survive repeated assembly, a threaded insert is usually more durable than tapping the base material.
UNC (coarse) is the default Unified series: quicker to assemble, more tolerant of dirt and damage, and better in soft materials such as aluminium and cast iron.
Clearance drills for a #4 screw: close fit #32 (0.1160 in, 2.95 mm), free fit #30 (0.1285 in, 3.26 mm).
FAQ
What size drill for a #4-40 UNC tap?
The standard tap drill for #4-40 UNC is a #43 drill, 0.0890 in (2.26 mm), which gives about 71% thread.
What clearance hole for a #4 UNC screw?
Use #32 (0.1160 in, 2.95 mm) for a close fit or #30 (0.1285 in, 3.26 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #4 screws.
What metric drill can I use for #4-40 UNC?
The closest metric drill is 2.25 mm (0.0886 in), giving about 72% thread on #4-40 UNC.
What is the difference between #4-40 UNC and #4-48 UNF?
Both are #4 diameter. #4-40 UNC has 40 threads per inch and uses a #43 tap drill (0.0890 in); #4-48 UNF has 48 TPI and uses a #42 drill (0.0935 in). The threads are not interchangeable.
Sources: ASME B1.1 — Unified inch screw threads (UN and UNR form) · Machinery's Handbook — tap drill sizes and percentage-of-thread formula · ASME B94.11M — twist drills (number and letter drill sizes) · Common machinist clearance-drill chart (close / free fit)
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