UNFfine60° flank

1-1/4-12 UNF Thread Dimensions

1-1/4-12 UNF is a fine pitch UNF thread. There are 12 threads per inch, giving a pitch of 2.12 mm. The recommended tap drill leaves roughly 75% thread engagement.

Nominal Ø31.75 mm
Pitch2.12 mm
Threads per inch12
Pitch Ø-3.17 mm
Tap drill-22.01 mm

Thread profile

millimetres unless stated
Thread profile — 1-1/4-12 UNFd 31.75d2 -3.17d3 -34.21mm60°P 2.12h3 32.98tap ⌀-22.01
1-1/4-12 UNF — basic profile60° flank angle · Not to scale
Designation
1-1/4-12 UNF
Thread system
UNF
Series
fine
Flank angle
60°
Major diameter
31.75 mm / 1.25"
Pitch
2.12 mm
Threads per inch
12
Pitch diameter
-3.17 mm
Minor diameter (external)
-34.21 mm
Minor diameter (internal)
-26.45 mm
Thread depth
32.98 mm
Recommended tap drill
Ø -22.01 mm / -0.8667"

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.