#8-36 UNF tap drill size
Use a #29 drill (0.1360 in, 3.45 mm) for #8-36 UNF. It is the standard chart size and leaves about 78% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #29 (chart drill) | 0.1360 | 3.45 | 77.6% | General purpose |
| 3.5 mm (metric drill) | 0.1378 | 3.5 | 72.6% | General purpose |
| #28 | 0.1405 | 3.57 | 65.1% | Hard or tough material, deep or blind holes |
| 9/64" | 0.1406 | 3.57 | 64.8% | Hard or tough material, deep or blind holes |
| #27 | 0.1440 | 3.66 | 55.4% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #5-44 UNF | #37 (0.1040 in) | 2.64 mm | #30 / #29 |
| #6-40 UNF | #33 (0.1130 in) | 2.87 mm | #27 / #25 |
| #8-36 UNF (this page) | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
| #10-32 UNF | #21 (0.1590 in) | 4.04 mm | #9 / #7 |
| #12-28 UNF | #14 (0.1820 in) | 4.62 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 0.1501 in (3.81 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 #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 #8-36 UNF tap?
The standard tap drill for #8-36 UNF is a #29 drill, 0.1360 in (3.45 mm), which gives about 78% thread.
What clearance hole for a #8 UNF 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 #8-36 UNF?
The closest metric drill is 3.5 mm (0.1378 in), giving about 73% thread on #8-36 UNF.
What is the difference between #8-36 UNF and #8-32 UNC?
Both are #8 diameter. #8-36 UNF has 36 threads per inch and uses a #29 tap drill (0.1360 in); #8-32 UNC has 32 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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