UNCcoarse60° flank

1-3/8-6 UNC Thread Dimensions

1-3/8-6 UNC is a coarse pitch UNC thread. There are 6 threads per inch, giving a pitch of 4.23 mm. The recommended tap drill leaves roughly 75% thread engagement.

Nominal Ø34.92 mm
Pitch4.23 mm
Threads per inch6
Pitch Ø-34.92 mm
Tap drill-72.6 mm

Thread profile

millimetres unless stated
Thread profile — 1-3/8-6 UNCd 34.92d2 -34.92d3 -97mm60°P 4.23h3 65.96tap ⌀-72.6
1-3/8-6 UNC — basic profile60° flank angle · Not to scale
Designation
1-3/8-6 UNC
Thread system
UNC
Series
coarse
Flank angle
60°
Major diameter
34.92 mm / 1.38"
Pitch
4.23 mm
Threads per inch
6
Pitch diameter
-34.92 mm
Minor diameter (external)
-97 mm
Minor diameter (internal)
-81.48 mm
Thread depth
65.96 mm
Recommended tap drill
Ø -72.6 mm / -2.8583"

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.