#2 UNC tap drill size
The tap drill for #2-56 UNC is 1.75 mm, which gives about 74% of full thread. There’s no single standard chart size, so this is worked out from the thread profile for 75%.
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Tap drill by thread percentage
75% is the usual workshop figure. Less thread is easier to tap; more adds little strength.
| Thread % | Ideal drill | Nearest metric drill | Nearest imperial drill |
|---|---|---|---|
| 60% | 1.831 mm 0.0721" | 1.85 mm (57%) | #49 0.0730" (56%) |
| 65% | 1.801 mm 0.0709" | 1.8 mm (65%) | #50 0.0700" (69%) |
| 70% | 1.772 mm 0.0698" | 1.75 mm (74%) | #50 0.0700" (69%) |
| 75% | 1.742 mm 0.0686" | 1.75 mm (74%) | #50 0.0700" (69%) |
| 80% | 1.713 mm 0.0674" | 1.7 mm (82%) | #51 0.0670" (82%) |
About this size
UNC is the general-purpose American inch thread. It shares the 60° basic profile with ISO metric, so the same tap-drill rule applies.
- Taps this small snap easily. By hand, or in stainless steel, a 1.8 mm drill (about 65% thread) makes tapping much easier for very little loss of strength.
- For a blind hole with 1.5 × D thread engagement (3.3 mm), drill about 5.1 mm deep. That leaves room for the 3–4 tapered lead threads on a second or plug tap. In steel, 1 × D engagement is usually enough; in aluminium or cast iron allow 1.5–2 × D.
Clearance holes for #2 UNC
| Fit | Drill |
|---|---|
| Close | 7/64" (2.78 mm) |
| Medium (normal) | 7/64" (2.78 mm) |
| Free (loose) | 1/8" (3.17 mm) |
Worked out with our own rule, not taken from a standards table: close = the larger of D + 4% and D + 0.2 mm, medium = the larger of D + 8% and D + 0.4 mm, free = the larger of D + 15% and D + 0.75 mm, each rounded up to the next 1/64". Check against your drawing where the fit matters.
#2 UNC thread data
| Designation | #2-56 UNC |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 56 TPI (0.454 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.086" (2.184 mm) |
| Pitch (effective) diameter | 0.074" (1.89 mm) |
| Minor diameter, external | 0.064" (1.628 mm) |
| Minor diameter, internal | 0.067" (1.693 mm) |
| Thread depth (internal) | 0.246 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
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Nearest alternatives
If you don’t have the right drill, these give:
- 1.7 mm drill → about 82% thread
- 1.75 mm drill → about 74% thread
- 1.8 mm drill → about 65% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- #2 UNF – 2.18 mm major, 64 TPI (0.397 mm pitch)
- 8BA – 2.2 mm major, 0.43 mm pitch (59.1 TPI equivalent)
- M2 – 2 mm major, 0.4 mm pitch (63.5 TPI equivalent)
Questions
What size drill for a #2 UNC tap?
The tap drill for #2-56 UNC is 1.75 mm, which gives about 74% of full thread. There’s no single standard chart size, so this is worked out from the thread profile for 75%.
What size clearance hole for #2 UNC?
For a #2 UNC screw or bolt, drill 7/64" for a close fit, 7/64" for a medium (normal) fit or 1/8" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of #2 UNC?
#2-56 UNC has a 56 TPI (0.454 mm pitch), a 60° thread angle and a 2.184 mm major diameter.
Sources: Basic sizes per ASME B1.1. Chart drill from the common UN 75% tapping chart where one exists. Number and letter drill sizes checked 25/10/2026. Values are for reference only – check your tap maker’s chart. See the disclaimer.
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