1"-8 UNC tap drill size
The standard tap drill for 1"-8 UNC is a 7/8" drill (0.8750 in, 22.23 mm), giving about 77% thread. At this size pilot-drill first and power-tap, thread-mill or single-point the thread.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 55/64" | 0.8594 | 21.83 | 86.6% | Tight: high tapping torque, tap breakage risk |
| 22.0 mm (metric drill) | 0.8661 | 22.0 | 82.4% | General purpose |
| 7/8" (chart drill) | 0.8750 | 22.23 | 77.0% | General purpose |
| 57/64" | 0.8906 | 22.62 | 67.4% | Hard or tough material, deep or blind holes |
| 29/32" | 0.9063 | 23.02 | 57.7% | Low engagement: check strength |
| 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 | 49/64" (0.7656 in) | 19.45 mm | 57/64" / 29/32" |
| 1"-8 UNC (this page) | 7/8" (0.8750 in) | 22.23 mm | 1-1/64" / 1-1/32" |
Practical notes
Drilling 7/8" (0.8750 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 0.9375 in (23.81 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 1" screw: close fit 1-1/64" (1.0156 in, 25.8 mm), free fit 1-1/32" (1.0313 in, 26.19 mm).
FAQ
What size drill for a 1"-8 UNC tap?
The standard tap drill for 1"-8 UNC is a 7/8" drill, 0.8750 in (22.23 mm), which gives about 77% thread.
What clearance hole for a 1" UNC screw?
Use 1-1/64" (1.0156 in, 25.8 mm) for a close fit or 1-1/32" (1.0313 in, 26.19 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 1" screws.
What metric drill can I use for 1"-8 UNC?
The closest metric drill is 22.0 mm (0.8661 in), giving about 82% thread on 1"-8 UNC.
What is the difference between 1"-8 UNC and 1"-12 UNF?
Both are 1" diameter. 1"-8 UNC has 8 threads per inch and uses a 7/8" tap drill (0.8750 in); 1"-12 UNF has 12 TPI and uses a 59/64" drill (0.9219 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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