7/16-20 UNF tap drill size
Use a 25/64" drill (0.3906 in, 9.92 mm) for 7/16-20 UNF. 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 |
|---|---|---|---|---|
| V | 0.3770 | 9.58 | 93.1% | Tight: high tapping torque, tap breakage risk |
| W | 0.3860 | 9.8 | 79.3% | General purpose |
| 9.9 mm (metric drill) | 0.3898 | 9.9 | 73.5% | General purpose |
| 25/64" (chart drill) | 0.3906 | 9.92 | 72.2% | General purpose |
| X | 0.3970 | 10.08 | 62.4% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 5/16-24 UNF | I (0.2720 in) | 6.91 mm | P / Q |
| 3/8-24 UNF | Q (0.3320 in) | 8.43 mm | W / X |
| 7/16-20 UNF (this page) | 25/64" (0.3906 in) | 9.92 mm | 29/64" / 15/32" |
| 1/2-20 UNF | 29/64" (0.4531 in) | 11.51 mm | 33/64" / 17/32" |
| 9/16-18 UNF | 33/64" (0.5156 in) | 13.1 mm | 37/64" / 19/32" |
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 0.4125 in (10.48 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 7/16 screw: close fit 29/64" (0.4531 in, 11.51 mm), free fit 15/32" (0.4688 in, 11.91 mm).
FAQ
What size drill for a 7/16-20 UNF tap?
The standard tap drill for 7/16-20 UNF is a 25/64" drill, 0.3906 in (9.92 mm), which gives about 72% thread.
What clearance hole for a 7/16 UNF screw?
Use 29/64" (0.4531 in, 11.51 mm) for a close fit or 15/32" (0.4688 in, 11.91 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 7/16 screws.
What metric drill can I use for 7/16-20 UNF?
The closest metric drill is 9.9 mm (0.3898 in), giving about 73% thread on 7/16-20 UNF.
What is the difference between 7/16-20 UNF and 7/16-14 UNC?
Both are 7/16 diameter. 7/16-20 UNF has 20 threads per inch and uses a 25/64" tap drill (0.3906 in); 7/16-14 UNC has 14 TPI and uses a U drill (0.3680 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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