7/8-9 UNC tap drill size
The standard tap drill for 7/8-9 UNC is a 49/64" drill (0.7656 in, 19.45 mm), giving about 76% thread. At this size pilot-drill first and power-tap, thread-mill or single-point the thread.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 3/4" | 0.7500 | 19.05 | 86.6% | Tight: high tapping torque, tap breakage risk |
| 49/64" (chart drill) | 0.7656 | 19.45 | 75.8% | General purpose |
| 19.5 mm (metric drill) | 0.7677 | 19.5 | 74.3% | General purpose |
| 25/32" | 0.7813 | 19.84 | 65.0% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 9/16-12 UNC | 31/64" (0.4844 in) | 12.3 mm | 37/64" / 19/32" |
| 5/8-11 UNC | 17/32" (0.5313 in) | 13.49 mm | 41/64" / 21/32" |
| 3/4-10 UNC | 21/32" (0.6563 in) | 16.67 mm | 49/64" / 25/32" |
| 7/8-9 UNC (this page) | 49/64" (0.7656 in) | 19.45 mm | 57/64" / 29/32" |
| 1"-8 UNC | 7/8" (0.8750 in) | 22.23 mm | 1-1/64" / 1-1/32" |
Practical notes
Drilling 49/64" (0.7656 in) is a heavy cut: pilot-drill, then open up, and check the hole size before tapping. An undersize hole is the usual cause of a seized tap at this size.
Use a tapping head, rigid tapping or thread milling on a machine. For one-off parts, single-point threading on a lathe or thread milling avoids the cost and risk of a large broken tap.
Thread-forming taps need about 0.8194 in (20.81 mm, D − 0.5 × pitch); that is approximate, so check the tap maker's chart.
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 7/8 screw: close fit 57/64" (0.8906 in, 22.62 mm), free fit 29/32" (0.9063 in, 23.02 mm).
FAQ
What size drill for a 7/8-9 UNC tap?
The standard tap drill for 7/8-9 UNC is a 49/64" drill, 0.7656 in (19.45 mm), which gives about 76% thread.
What clearance hole for a 7/8 UNC screw?
Use 57/64" (0.8906 in, 22.62 mm) for a close fit or 29/32" (0.9063 in, 23.02 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 7/8 screws.
What metric drill can I use for 7/8-9 UNC?
The closest metric drill is 19.5 mm (0.7677 in), giving about 74% thread on 7/8-9 UNC.
What is the difference between 7/8-9 UNC and 7/8-14 UNF?
Both are 7/8 diameter. 7/8-9 UNC has 9 threads per inch and uses a 49/64" tap drill (0.7656 in); 7/8-14 UNF has 14 TPI and uses a 13/16" drill (0.8125 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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