5/16-18 UNC Thread Dimensions
5/16-18 UNC is a Unified National Coarse thread. There are 18 threads per inch on a 0.3125 in major diameter, giving a metric-equivalent pitch of 1.411 mm. The recommended tap drill is nominal diameter minus pitch.
Thread profile
millimetres unless statedBasic information
- Designation
- 5/16-18 UNC
- Thread system
- Unified National Coarse
- Series
- coarse
- Standard
- ASME B1.1
- Basic profile
- Unified basic profile
- Flank angle
- 60°
Dimensions
- Major diameter
- 0.3125 in / 7.938 mm
- Pitch (metric equivalent)
- 1.411 mm
- Threads per inch
- 18
- Pitch diameter
- 0.2764 in / 7.02 mm
- Minor diameter (external)
- 0.2445 in / 6.21 mm
- Minor diameter (internal)
- 0.2524 in / 6.41 mm
- Thread depth
- 0.0341 in / 0.866 mm
- Recommended tap drill
- Ø 0.2569 in / 6.526 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 has a fundamental triangle height H = 0.866025 × P, with pitch diameter d₂ = d − 0.649519 × P, external minor diameter d₃ = d − 1.226869 × P and internal minor diameter D₁ = d − 1.082532 × P; the two minor diameters differ because the external root is rounded while the internal crest is truncated. 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 parts16 specification pages
All 5/16-18 fastenersEvery fastener below is threaded 5/16-18 UNC, so a nut, bolt or washer picked from this list will mate with it.
The dash in the Length column marks a nut or a washer: those parts have no length to quote, so the dash means not applicable rather than missing data.
Related
Neighbouring sizes in Unified National Coarse.