UNCcoarse60° flank
5/16-18 UNC Thread Dimensions
5/16-18 UNC is a coarse pitch UNC thread. There are 18 threads per inch, giving a pitch of 1.41 mm. The recommended tap drill leaves roughly 75% thread engagement.
Nominal Ø7.94 mm
Pitch1.41 mm
Threads per inch18
Pitch Ø-15.34 mm
Tap drill-27.9 mm
Thread profile
millimetres unless stated- Designation
- 5/16-18 UNC
- Thread system
- UNC
- Series
- coarse
- Flank angle
- 60°
- Major diameter
- 7.94 mm / 0.31"
- Pitch
- 1.41 mm
- Threads per inch
- 18
- Pitch diameter
- -15.34 mm
- Minor diameter (external)
- -36.04 mm
- Minor diameter (internal)
- -30.86 mm
- Thread depth
- 21.99 mm
- Recommended tap drill
- Ø -27.9 mm / -1.0986"
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.