UNCcoarse60° flank
1-8 UNC Thread Dimensions
1-8 UNC is a coarse pitch UNC thread. There are 8 threads per inch, giving a pitch of 3.17 mm. The recommended tap drill leaves roughly 75% thread engagement.
Nominal Ø25.4 mm
Pitch3.17 mm
Threads per inch8
Pitch Ø-26.98 mm
Tap drill-55.24 mm
Thread profile
millimetres unless stated- Designation
- 1-8 UNC
- Thread system
- UNC
- Series
- coarse
- Flank angle
- 60°
- Major diameter
- 25.4 mm / 1"
- Pitch
- 3.17 mm
- Threads per inch
- 8
- Pitch diameter
- -26.98 mm
- Minor diameter (external)
- -73.54 mm
- Minor diameter (internal)
- -61.9 mm
- Thread depth
- 49.47 mm
- Recommended tap drill
- Ø -55.24 mm / -2.175"
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.