7/8-20 UNEF Thread Dimensions
7/8-20 UNEF is a extra fine Unified National Extra Fine thread. There are 20 threads per inch on a 0.8748 in major diameter, giving a metric-equivalent pitch of 1.27 mm. The recommended tap drill leaves roughly 75% thread engagement.
Thread profile
millimetres unless statedBasic information
- Designation
- 7/8-20 UNEF
- Thread system
- Unified National Extra Fine
- Series
- extra fine
- Standard
- ASME B1.1
- Basic profile
- Unified basic profile
- Flank angle
- 60°
Dimensions
- Major diameter
- 0.8748 in / 22.22 mm
- Pitch (metric equivalent)
- 1.27 mm
- Threads per inch
- 20
- Pitch diameter
- 0.8425 in / 21.4 mm
- Minor diameter (external)
- 0.8138 in / 20.67 mm
- Minor diameter (internal)
- 0.8209 in / 20.85 mm
- Thread depth
- 0.0307 in / 0.779 mm
- Recommended tap drill
- Ø 0.8248 in / 20.95 mm
How these values are derived
computed from the basic profileThese figures follow the Unified basic profile of ASME B1.1. A 60° basic profile uses a fundamental triangle height H = 0.866025 × P, with external minor diameter d₃ = d − 1.226869 × P and internal minor diameter D₁ = d − 1.082532 × P; the two differ because the external root is rounded while the internal crest is truncated. A 55° Whitworth profile uses H = 0.960491 × P and a thread height of 0.640327 × P. The recommended tap drill is given as nominal diameter minus pitch, which approximates 75% thread engagement — adjust it for tough materials or deep holes.
Compatible parts0 specification pages
No specification in this database uses 7/8-20 UNEF yet.
Related
Neighbouring sizes in Unified National Extra Fine.