UNC — Unified coarse

#5-40 UNC

Tap drill #38 for a #5-40 UNC thread — major Ø 3.175 mm, pitch 0.635 mm (40 TPI).

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Tap drill

#38

2.58 mm · ≈72% thread

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Thread form & key diameters

Dimensions

Designation	#5-40 UNC
Major (outside) diameter	3.175 mm · 0.125"
Pitch	0.635 mm · 40 TPI
Pitch diameter	2.763 mm · 0.1088"
Minor diameter (external)	2.396 mm · 0.0943"
Minor diameter (internal)	2.488 mm · 0.0979"
Recommended tap drill	#38 · 2.58 mm · ≈72% thread
Clearance hole	Close 9/64" · Normal 5/32" · Loose 3/16" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Socket key (across flats)	3/32"

Close 9/64" · Normal 5/32" · Loose 3/16" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).

What tap drill do I need for #5-40 UNC?

For a #5-40 UNC thread, drill the hole with a #38 tap drill (2.58 mm) before tapping. That produces roughly 72% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 3.175 mm and a pitch of 0.635 mm (40 TPI).

To pass a #5-40 UNC bolt through a part rather than thread it, use a clearance hole: Close 9/64" · Normal 5/32" · Loose 3/16" — 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 #5 threads

#5-44 UNFTap #37Ø 3.175 mm · pitch 0.577 mm

Nearby sizes

More unc — unified coarse threads

#4-40 UNCTap #43Ø 2.845 mm · pitch 0.635 mm #6-32 UNCTap #36Ø 3.505 mm · pitch 0.794 mm #3-48 UNCTap #47Ø 2.515 mm · pitch 0.529 mm #2-56 UNCTap #50Ø 2.184 mm · pitch 0.454 mm #8-32 UNCTap #29Ø 4.166 mm · pitch 0.794 mm #1-64 UNCTap #53Ø 1.854 mm · pitch 0.397 mm #10-24 UNCTap #25Ø 4.826 mm · pitch 1.058 mm #12-24 UNCTap #16Ø 5.486 mm · pitch 1.058 mm