5/8-11 UNC tap drill size
The standard tap drill for 5/8-11 UNC is a 17/32" drill (0.5313 in, 13.49 mm), giving about 79% thread. At this size pilot-drill first and power-tap, thread-mill or single-point the thread.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 33/64" | 0.5156 | 13.1 | 92.6% | Tight: high tapping torque, tap breakage risk |
| 17/32" (chart drill) | 0.5313 | 13.49 | 79.4% | General purpose |
| 13.5 mm (metric drill) | 0.5315 | 13.5 | 79.2% | General purpose |
| 35/64" | 0.5469 | 13.89 | 66.2% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 1/2-13 UNC | 27/64" (0.4219 in) | 10.72 mm | 33/64" / 17/32" |
| 9/16-12 UNC | 31/64" (0.4844 in) | 12.3 mm | 37/64" / 19/32" |
| 5/8-11 UNC (this page) | 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" |
Practical notes
Drilling 17/32" (0.5313 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 0.5795 in (14.72 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 5/8 screw: close fit 41/64" (0.6406 in, 16.27 mm), free fit 21/32" (0.6563 in, 16.67 mm).
FAQ
What size drill for a 5/8-11 UNC tap?
The standard tap drill for 5/8-11 UNC is a 17/32" drill, 0.5313 in (13.49 mm), which gives about 79% thread.
What clearance hole for a 5/8 UNC screw?
Use 41/64" (0.6406 in, 16.27 mm) for a close fit or 21/32" (0.6563 in, 16.67 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 5/8 screws.
What metric drill can I use for 5/8-11 UNC?
The closest metric drill is 13.5 mm (0.5315 in), giving about 79% thread on 5/8-11 UNC.
What is the difference between 5/8-11 UNC and 5/8-18 UNF?
Both are 5/8 diameter. 5/8-11 UNC has 11 threads per inch and uses a 17/32" tap drill (0.5313 in); 5/8-18 UNF has 18 TPI and uses a 37/64" drill (0.5781 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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