UNFfine60° flank
5/8-18 UNF Thread Dimensions
5/8-18 UNF is a fine pitch UNF thread. There are 18 threads per inch, giving a pitch of 1.41 mm. The recommended tap drill leaves roughly 75% thread engagement.
Nominal Ø15.88 mm
Pitch1.41 mm
Threads per inch18
Pitch Ø-7.41 mm
Tap drill-19.97 mm
Thread profile
millimetres unless stated- Designation
- 5/8-18 UNF
- Thread system
- UNF
- Series
- fine
- Flank angle
- 60°
- Major diameter
- 15.88 mm / 0.62"
- Pitch
- 1.41 mm
- Threads per inch
- 18
- Pitch diameter
- -7.41 mm
- Minor diameter (external)
- -28.1 mm
- Minor diameter (internal)
- -22.93 mm
- Thread depth
- 21.99 mm
- Recommended tap drill
- Ø -19.97 mm / -0.7861"
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.