UNFfine60° flank

1/4-28 UNF Thread Dimensions

1/4-28 UNF is a fine pitch UNF thread. There are 28 threads per inch, giving a pitch of 0.91 mm. The recommended tap drill leaves roughly 75% thread engagement.

Nominal Ø6.35 mm
Pitch0.91 mm
Threads per inch28
Pitch Ø-8.62 mm
Tap drill-16.69 mm

Thread profile

millimetres unless stated
Thread profile — 1/4-28 UNFd 6.35d2 -8.62d3 -21.92mm60°P 0.91h3 14.13tap ⌀-16.69
1/4-28 UNF — basic profile60° flank angle · Not to scale
Designation
1/4-28 UNF
Thread system
UNF
Series
fine
Flank angle
60°
Major diameter
6.35 mm / 0.25"
Pitch
0.91 mm
Threads per inch
28
Pitch diameter
-8.62 mm
Minor diameter (external)
-21.92 mm
Minor diameter (internal)
-18.59 mm
Thread depth
14.13 mm
Recommended tap drill
Ø -16.69 mm / -0.6571"

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.