#8-32 UNC tap drill size
Use a #29 drill (0.1360 in, 3.45 mm) for #8-32 UNC. It is the standard chart size and leaves about 69% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #30 | 0.1285 | 3.26 | 87.4% | Tight: high tapping torque, tap breakage risk |
| #29 (chart drill) | 0.1360 | 3.45 | 69.0% | Hard or tough material, deep or blind holes |
| 3.5 mm (metric drill) | 0.1378 | 3.5 | 64.6% | Hard or tough material, deep or blind holes |
| #28 | 0.1405 | 3.57 | 57.9% | Low engagement: check strength |
| 9/64" | 0.1406 | 3.57 | 57.6% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #5-40 UNC | #38 (0.1015 in) | 2.58 mm | #30 / #29 |
| #6-32 UNC | #36 (0.1065 in) | 2.71 mm | #27 / #25 |
| #8-32 UNC (this page) | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
| #10-24 UNC | #25 (0.1495 in) | 3.8 mm | #9 / #7 |
| #12-24 UNC | #16 (0.1770 in) | 4.5 mm | #2 / #1 |
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 0.1484 in (3.77 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 #8 screw: close fit #18 (0.1695 in, 4.31 mm), free fit #16 (0.1770 in, 4.5 mm).
FAQ
What size drill for a #8-32 UNC tap?
The standard tap drill for #8-32 UNC is a #29 drill, 0.1360 in (3.45 mm), which gives about 69% thread.
What clearance hole for a #8 UNC screw?
Use #18 (0.1695 in, 4.31 mm) for a close fit or #16 (0.1770 in, 4.5 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #8 screws.
What metric drill can I use for #8-32 UNC?
The closest metric drill is 3.5 mm (0.1378 in), giving about 65% thread on #8-32 UNC.
What is the difference between #8-32 UNC and #8-36 UNF?
Both are #8 diameter. #8-32 UNC has 32 threads per inch and uses a #29 tap drill (0.1360 in); #8-36 UNF has 36 TPI and uses a #29 drill (0.1360 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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