UNC — Unified coarse

#4-40 UNC

Tap drill #43 for a #4-40 UNC thread — major Ø 2.845 mm, pitch 0.635 mm (40 TPI).

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Tap drill

#43

2.26 mm · ≈71% thread

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Thread form & key diameters

Dimensions

Designation	#4-40 UNC
Major (outside) diameter	2.845 mm · 0.112"
Pitch	0.635 mm · 40 TPI
Pitch diameter	2.432 mm · 0.0958"
Minor diameter (external)	2.066 mm · 0.0813"
Minor diameter (internal)	2.157 mm · 0.0849"
Recommended tap drill	#43 · 2.26 mm · ≈71% thread
Clearance hole	Close #30 · Normal #27 · Loose #17 — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Socket key (across flats)	3/32"

Close #30 · Normal #27 · Loose #17 — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).

What tap drill do I need for #4-40 UNC?

For a #4-40 UNC thread, drill the hole with a #43 tap drill (2.26 mm) before tapping. That produces roughly 71% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 2.845 mm and a pitch of 0.635 mm (40 TPI).

To pass a #4-40 UNC bolt through a part rather than thread it, use a clearance hole: Close #30 · Normal #27 · Loose #17 — 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 #4 threads

#4-48 UNFTap #42Ø 2.845 mm · pitch 0.529 mm

Nearby sizes

More unc — unified coarse threads

#5-40 UNCTap #38Ø 3.175 mm · pitch 0.635 mm #3-48 UNCTap #47Ø 2.515 mm · pitch 0.529 mm #6-32 UNCTap #36Ø 3.505 mm · pitch 0.794 mm #2-56 UNCTap #50Ø 2.184 mm · pitch 0.454 mm #1-64 UNCTap #53Ø 1.854 mm · pitch 0.397 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