1/4-20 UNC thread: 6.35 mm × 20 TPI
Threads › 1/4-20 UNC
1/4-20 UNC is a UNC thread: 6.35 mm major diameter, 20 TPI (1.27 mm pitch), 60° form. Tap drill 5.1 mm.
| Standard | UNC |
|---|---|
| Major diameter | 6.35 mm (0.25″) |
| Pitch | 20 TPI (1.27 mm) |
| Thread angle | 60° |
| Minor diameter, bolt | 4.79 mm |
| Minor diameter, nut | 4.98 mm |
| Tap drill | 5.1 mm |
| Usual class | 2A bolt, 2B nut |
Minor diameters worked out from the basic profile: major Ø − 1.22687 × pitch (bolt) and − 1.08253 × pitch (nut).
Open 1/4-20 UNC in BenchsmithAdd to order list
Measure it
Bolt: Measure across the crests of the bolt with the caliper jaws. Worn bolts read 0.1 to 0.3 under.
Nut or tapped hole: Measure across the tops of the threads inside the nut or hole with the inside jaws. That is the minor Ø. Worn or oversize holes read a little over.
Same diameter, other pitch
- 1/4-28 UNF28 TPI (0.907 mm pitch)
- 1/4 BSW20 TPI (1.27 mm pitch)
Close in size, won't mate
- 1/4-16 ACMEØ6.35 mm, 16 TPI
- M6Ø6 mm, 1 mm pitch
- M6×0.75Ø6 mm, 0.75 mm pitch
One size up or down
- #12-24 UNCØ5.49 mm (down)
- 5/16-18 UNCØ7.94 mm (up)
Got a bolt and no idea what it is? Measure the outside Ø and type it into Benchsmith → Find → Thread.