UNCcoarse60° flank

1-1/4-7 UNC Thread Dimensions

1-1/4-7 UNC is a coarse pitch UNC thread. There are 7 threads per inch, giving a pitch of 3.63 mm. The recommended tap drill leaves roughly 75% thread engagement.

Nominal Ø31.75 mm
Pitch3.63 mm
Threads per inch7
Pitch Ø-28.11 mm
Tap drill-60.42 mm

Thread profile

millimetres unless stated
Thread profile — 1-1/4-7 UNCd 31.75d2 -28.11d3 -81.33mm60°P 3.63h3 56.54tap ⌀-60.42
1-1/4-7 UNC — basic profile60° flank angle · Not to scale
Designation
1-1/4-7 UNC
Thread system
UNC
Series
coarse
Flank angle
60°
Major diameter
31.75 mm / 1.25"
Pitch
3.63 mm
Threads per inch
7
Pitch diameter
-28.11 mm
Minor diameter (external)
-81.33 mm
Minor diameter (internal)
-68.02 mm
Thread depth
56.54 mm
Recommended tap drill
Ø -60.42 mm / -2.3786"

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.