UNCcoarse60° flank

7/8-9 UNC Thread Dimensions

7/8-9 UNC is a coarse pitch UNC thread. There are 9 threads per inch, giving a pitch of 2.82 mm. The recommended tap drill leaves roughly 75% thread engagement.

Nominal Ø22.22 mm
Pitch2.82 mm
Threads per inch9
Pitch Ø-24.34 mm
Tap drill-49.46 mm

Thread profile

millimetres unless stated
Thread profile — 7/8-9 UNCd 22.22d2 -24.34d3 -65.72mm60°P 2.82h3 43.97tap ⌀-49.46
7/8-9 UNC — basic profile60° flank angle · Not to scale
Designation
7/8-9 UNC
Thread system
UNC
Series
coarse
Flank angle
60°
Major diameter
22.22 mm / 0.88"
Pitch
2.82 mm
Threads per inch
9
Pitch diameter
-24.34 mm
Minor diameter (external)
-65.72 mm
Minor diameter (internal)
-55.38 mm
Thread depth
43.97 mm
Recommended tap drill
Ø -49.46 mm / -1.9472"

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.