UNF — Unified fine

#6-40 UNF

Tap drill #33 for a #6-40 UNF thread — major Ø 3.505 mm, pitch 0.635 mm (40 TPI).

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Tap drill

#33

2.87 mm · ≈77% thread

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Thread form & key diameters

Dimensions

Designation	#6-40 UNF
Major (outside) diameter	3.505 mm · 0.138"
Pitch	0.635 mm · 40 TPI
Pitch diameter	3.093 mm · 0.1218"
Minor diameter (external)	2.726 mm · 0.1073"
Minor diameter (internal)	2.818 mm · 0.1109"
Recommended tap drill	#33 · 2.87 mm · ≈77% thread
Clearance hole	Close #23 · Normal #18 · Loose #7 — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Socket key (across flats)	7/64"

Close #23 · Normal #18 · Loose #7 — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).

What tap drill do I need for #6-40 UNF?

For a #6-40 UNF thread, drill the hole with a #33 tap drill (2.87 mm) before tapping. That produces roughly 77% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 3.505 mm and a pitch of 0.635 mm (40 TPI).

To pass a #6-40 UNF bolt through a part rather than thread it, use a clearance hole: Close #23 · Normal #18 · Loose #7 — 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 #6 threads

M3.5 × 0.6Tap 2.9 mmØ 3.5 mm · pitch 0.6 mm #6-32 UNCTap #36Ø 3.505 mm · pitch 0.794 mm

Nearby sizes

More unf — unified fine threads

#5-44 UNFTap #37Ø 3.175 mm · pitch 0.577 mm #4-48 UNFTap #42Ø 2.845 mm · pitch 0.529 mm #8-36 UNFTap #29Ø 4.166 mm · pitch 0.706 mm #3-56 UNFTap #45Ø 2.515 mm · pitch 0.454 mm #2-64 UNFTap #50Ø 2.184 mm · pitch 0.397 mm #10-32 UNFTap #21Ø 4.826 mm · pitch 0.794 mm #1-72 UNFTap #53Ø 1.854 mm · pitch 0.353 mm #0-80 UNFTap 3/64"Ø 1.524 mm · pitch 0.318 mm