7/8-14 UNF tap drill size
The standard tap drill for 7/8-14 UNF is a 13/16" drill (0.8125 in, 20.64 mm), giving about 67% thread. At this size pilot-drill first and power-tap, thread-mill or single-point the thread.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 51/64" | 0.7969 | 20.24 | 84.2% | General purpose |
| 20.5 mm (metric drill) | 0.8071 | 20.5 | 73.2% | General purpose |
| 13/16" (chart drill) | 0.8125 | 20.64 | 67.4% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 9/16-18 UNF | 33/64" (0.5156 in) | 13.1 mm | 37/64" / 19/32" |
| 5/8-18 UNF | 37/64" (0.5781 in) | 14.68 mm | 41/64" / 21/32" |
| 3/4-16 UNF | 11/16" (0.6875 in) | 17.46 mm | 49/64" / 25/32" |
| 7/8-14 UNF (this page) | 13/16" (0.8125 in) | 20.64 mm | 57/64" / 29/32" |
| 1"-12 UNF | 59/64" (0.9219 in) | 23.42 mm | 1-1/64" / 1-1/32" |
Practical notes
Drilling 13/16" (0.8125 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 0.8393 in (21.32 mm, D − 0.5 × pitch); that is approximate, so check the tap maker's chart.
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 7/8 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 7/8-14 UNF tap?
The standard tap drill for 7/8-14 UNF is a 13/16" drill, 0.8125 in (20.64 mm), which gives about 67% thread.
What clearance hole for a 7/8 UNF 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 7/8-14 UNF?
The closest metric drill is 20.5 mm (0.8071 in), giving about 73% thread on 7/8-14 UNF.
What is the difference between 7/8-14 UNF and 7/8-9 UNC?
Both are 7/8 diameter. 7/8-14 UNF has 14 threads per inch and uses a 13/16" tap drill (0.8125 in); 7/8-9 UNC has 9 TPI and uses a 49/64" drill (0.7656 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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