UNFfine60° flank

#6-40 UNF Thread Dimensions

#6-40 UNF is a fine pitch UNF thread. There are 40 threads per inch, giving a pitch of 0.64 mm. The recommended tap drill leaves roughly 75% thread engagement.

Nominal Ø3.51 mm
Pitch0.64 mm
Threads per inch40
Pitch Ø-6.97 mm
Tap drill-12.62 mm

Thread profile

millimetres unless stated
Thread profile — #6-40 UNFd 3.51d2 -6.97d3 -16.28mm60°P 0.64h3 9.89tap ⌀-12.62
#6-40 UNF — basic profile60° flank angle · Not to scale
Designation
#6-40 UNF
Thread system
UNF
Series
fine
Flank angle
60°
Major diameter
3.51 mm / 0.14"
Pitch
0.64 mm
Threads per inch
40
Pitch diameter
-6.97 mm
Minor diameter (external)
-16.28 mm
Minor diameter (internal)
-13.95 mm
Thread depth
9.89 mm
Recommended tap drill
Ø -12.62 mm / -0.497"

How these values are derived

Basic ISO thread geometry uses the fundamental triangle height H = 0.866025 × P. External minor diameter d₃ = d − 1.226869 × P, internal minor diameter D₁ = d − 1.082532 × P. Tap drill is given as nominal diameter minus pitch, which approximates a 75% thread engagement — adjust for tough materials or deep holes.