7/8-9 UNC Thread Dimensions
7/8-9 UNC is a coarse Unified National Coarse thread. There are 9 threads per inch on a 0.8748 in major diameter, giving a metric-equivalent pitch of 2.822 mm. The recommended tap drill leaves roughly 75% thread engagement.
Thread profile
millimetres unless statedBasic information
- Designation
- 7/8-9 UNC
- Thread system
- Unified National Coarse
- Series
- coarse
- Standard
- ASME B1.1
- Basic profile
- Unified basic profile
- Flank angle
- 60°
Dimensions
- Major diameter
- 0.8748 in / 22.22 mm
- Pitch (metric equivalent)
- 2.822 mm
- Threads per inch
- 9
- Pitch diameter
- 0.8028 in / 20.39 mm
- Minor diameter (external)
- 0.7386 in / 18.76 mm
- Minor diameter (internal)
- 0.7547 in / 19.17 mm
- Thread depth
- 0.0681 in / 1.731 mm
- Recommended tap drill
- Ø 0.7638 in / 19.4 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 parts7 specification pages
All 7/8-9 fastenersEvery fastener below is threaded 7/8-9 UNC, so a nut, bolt or washer picked from this list will mate with it.
Related
Neighbouring sizes in Unified National Coarse.