#5-40 UNC tap drill size
Use a #38 drill (0.1015 in, 2.58 mm) for #5-40 UNC. It is the standard chart size and leaves about 72% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #41 | 0.0960 | 2.44 | 89.3% | Tight: high tapping torque, tap breakage risk |
| #40 | 0.0980 | 2.49 | 83.1% | General purpose |
| #39 | 0.0995 | 2.53 | 78.5% | General purpose |
| #38 (chart drill) | 0.1015 | 2.58 | 72.4% | General purpose |
| 2.6 mm (metric drill) | 0.1024 | 2.6 | 69.7% | Hard or tough material, deep or blind holes |
| #37 | 0.1040 | 2.64 | 64.7% | Hard or tough material, deep or blind holes |
| #36 | 0.1065 | 2.71 | 57.0% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #3-48 UNC | #47 (0.0785 in) | 1.99 mm | #37 / #35 |
| #4-40 UNC | #43 (0.0890 in) | 2.26 mm | #32 / #30 |
| #5-40 UNC (this page) | #38 (0.1015 in) | 2.58 mm | #30 / #29 |
| #6-32 UNC | #36 (0.1065 in) | 2.71 mm | #27 / #25 |
| #8-32 UNC | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
Practical notes
Spiral-point taps suit through holes; spiral-flute taps lift chips out of blind holes. In a blind hole, drill a few threads deeper than the full-thread depth you need.
Typical engagement lengths: about 1 × diameter in steel, 1.5 × in cast iron or brass and 2 × in aluminium. In tough stainless or hardened steel a slightly larger drill (60–70% thread) cuts torque and tap breakage with little loss of strength.
Thread-forming (roll) taps need a larger hole, roughly 0.1125 in (2.86 mm, D − 0.5 × pitch). Check the tap maker's chart; forming suits ductile metals such as aluminium, mild steel and brass, not cast iron.
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 #5 screw: close fit #30 (0.1285 in, 3.26 mm), free fit #29 (0.1360 in, 3.45 mm).
FAQ
What size drill for a #5-40 UNC tap?
The standard tap drill for #5-40 UNC is a #38 drill, 0.1015 in (2.58 mm), which gives about 72% thread.
What clearance hole for a #5 UNC screw?
Use #30 (0.1285 in, 3.26 mm) for a close fit or #29 (0.1360 in, 3.45 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #5 screws.
What metric drill can I use for #5-40 UNC?
The closest metric drill is 2.6 mm (0.1024 in), giving about 70% thread on #5-40 UNC.
What is the difference between #5-40 UNC and #5-44 UNF?
Both are #5 diameter. #5-40 UNC has 40 threads per inch and uses a #38 tap drill (0.1015 in); #5-44 UNF has 44 TPI and uses a #37 drill (0.1040 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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