#2-56 UNC tap drill size
Tap #2-56 UNC with a #50 drill (0.0700 in, 1.78 mm), about 69% thread. Taps this fine break easily, so hand-tap with cutting oil and back off often.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #51 | 0.0670 | 1.7 | 81.9% | General purpose |
| #50 (chart drill) | 0.0700 | 1.78 | 69.0% | Hard or tough material, deep or blind holes |
| 1.8 mm (metric drill) | 0.0709 | 1.8 | 65.2% | Hard or tough material, deep or blind holes |
| #49 | 0.0730 | 1.85 | 56.0% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #1-64 UNC | #53 (0.0595 in) | 1.51 mm | #48 / #46 |
| #2-56 UNC (this page) | #50 (0.0700 in) | 1.78 mm | #43 / #41 |
| #3-48 UNC | #47 (0.0785 in) | 1.99 mm | #37 / #35 |
| #4-40 UNC | #43 (0.0890 in) | 2.26 mm | #32 / #30 |
| #5-40 UNC | #38 (0.1015 in) | 2.58 mm | #30 / #29 |
Practical notes
At 56 TPI the basic thread depth is only about 0.0097 in per side, so small errors in drill size change engagement a lot. Use a sharp, correctly sized drill, a tap guide to keep the tap square, and limit thread depth to about 1.5 × diameter.
Thread-forming taps are popular for #2 sizes in aluminium and brass because they make no chips; they need a larger hole, roughly 0.0771 in (1.96 mm, D − 0.5 × pitch). Confirm against the tap maker's chart.
If a #2 thread in plastic or a printed part must survive repeated assembly, a threaded insert is usually more durable than tapping the base material.
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 #2 screw: close fit #43 (0.0890 in, 2.26 mm), free fit #41 (0.0960 in, 2.44 mm).
FAQ
What size drill for a #2-56 UNC tap?
The standard tap drill for #2-56 UNC is a #50 drill, 0.0700 in (1.78 mm), which gives about 69% thread.
What clearance hole for a #2 UNC screw?
Use #43 (0.0890 in, 2.26 mm) for a close fit or #41 (0.0960 in, 2.44 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #2 screws.
What metric drill can I use for #2-56 UNC?
The closest metric drill is 1.8 mm (0.0709 in), giving about 65% thread on #2-56 UNC.
What is the difference between #2-56 UNC and #2-64 UNF?
Both are #2 diameter. #2-56 UNC has 56 threads per inch and uses a #50 tap drill (0.0700 in); #2-64 UNF has 64 TPI and uses a #50 drill (0.0700 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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