#6-40 UNF tap drill size
Use a #33 drill (0.1130 in, 2.87 mm) for #6-40 UNF. It is the standard chart size and leaves about 77% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 7/64" | 0.1094 | 2.78 | 88.1% | Tight: high tapping torque, tap breakage risk |
| #35 | 0.1100 | 2.79 | 86.2% | Tight: high tapping torque, tap breakage risk |
| #34 | 0.1110 | 2.82 | 83.1% | General purpose |
| 2.85 mm (metric drill) | 0.1122 | 2.85 | 79.4% | General purpose |
| #33 (chart drill) | 0.1130 | 2.87 | 77.0% | General purpose |
| #32 | 0.1160 | 2.95 | 67.7% | Hard or tough material, deep or blind holes |
| #31 | 0.1200 | 3.05 | 55.4% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #4-48 UNF | #42 (0.0935 in) | 2.37 mm | #32 / #30 |
| #5-44 UNF | #37 (0.1040 in) | 2.64 mm | #30 / #29 |
| #6-40 UNF (this page) | #33 (0.1130 in) | 2.87 mm | #27 / #25 |
| #8-36 UNF | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
| #10-32 UNF | #21 (0.1590 in) | 4.04 mm | #9 / #7 |
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 0.1255 in (3.19 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.
UNF (fine) threads have more threads per inch than UNC, a larger tensile stress area and better resistance to loosening, but they strip more easily in soft material and are more sensitive to drill size.
Clearance drills for a #6 screw: close fit #27 (0.1440 in, 3.66 mm), free fit #25 (0.1495 in, 3.8 mm).
FAQ
What size drill for a #6-40 UNF tap?
The standard tap drill for #6-40 UNF is a #33 drill, 0.1130 in (2.87 mm), which gives about 77% thread.
What clearance hole for a #6 UNF screw?
Use #27 (0.1440 in, 3.66 mm) for a close fit or #25 (0.1495 in, 3.8 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #6 screws.
What metric drill can I use for #6-40 UNF?
The closest metric drill is 2.85 mm (0.1122 in), giving about 79% thread on #6-40 UNF.
What is the difference between #6-40 UNF and #6-32 UNC?
Both are #6 diameter. #6-40 UNF has 40 threads per inch and uses a #33 tap drill (0.1130 in); #6-32 UNC has 32 TPI and uses a #36 drill (0.1065 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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