UNC — Unified coarse

#2-56 UNC

Tap drill #50 for a #2-56 UNC thread — major Ø 2.184 mm, pitch 0.454 mm (56 TPI).

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Tap drill

#50

1.78 mm · ≈69% thread

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Thread form & key diameters

Dimensions

Designation	#2-56 UNC
Major (outside) diameter	2.184 mm · 0.086"
Pitch	0.454 mm · 56 TPI
Pitch diameter	1.89 mm · 0.0744"
Minor diameter (external)	1.628 mm · 0.0641"
Minor diameter (internal)	1.693 mm · 0.0667"
Recommended tap drill	#50 · 1.78 mm · ≈69% thread
Clearance hole	Close #38 · Normal #32 · Loose #25 — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Socket key (across flats)	5/64"

Close #38 · Normal #32 · Loose #25 — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).

What tap drill do I need for #2-56 UNC?

For a #2-56 UNC thread, drill the hole with a #50 tap drill (1.78 mm) before tapping. That produces roughly 69% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 2.184 mm and a pitch of 0.454 mm (56 TPI).

To pass a #2-56 UNC bolt through a part rather than thread it, use a clearance hole: Close #38 · Normal #32 · Loose #25 — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice)..

All values are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables — not hand-entered. See the full reference.

Same diameter

Other #2 threads

#2-64 UNFTap #50Ø 2.184 mm · pitch 0.397 mm

Nearby sizes

More unc — unified coarse threads

#1-64 UNCTap #53Ø 1.854 mm · pitch 0.397 mm #3-48 UNCTap #47Ø 2.515 mm · pitch 0.529 mm #4-40 UNCTap #43Ø 2.845 mm · pitch 0.635 mm #5-40 UNCTap #38Ø 3.175 mm · pitch 0.635 mm #6-32 UNCTap #36Ø 3.505 mm · pitch 0.794 mm #8-32 UNCTap #29Ø 4.166 mm · pitch 0.794 mm #10-24 UNCTap #25Ø 4.826 mm · pitch 1.058 mm #12-24 UNCTap #16Ø 5.486 mm · pitch 1.058 mm