Example designation
0.2500-20 UNC-2A
- 0.2500
- Nominal major diameter in inches.
- 20
- Threads per inch; the pitch is 0.0500 in.
- UNC
- Unified coarse series.
- 2A
- Class 2 external thread. The corresponding internal selection is 2B.
Series follows the diameter and pitch
Automatic series selection checks the complete size against the catalog. Coarse, fine, extra fine, and constant-pitch series have specific combinations; a custom combination receives a special-series designation.
UN, UNR, and UNJ
All three use a 60° profile, but their root requirements differ. UNR specifies a rounded external root; UNJ has its own root and mating-profile requirements. Select the profile on the drawing before choosing the fit.
Allow for the finish
A and B identify external and internal threads. When planning a coating, select the treated side and compare the finished pitch-diameter limits with the required sizes before coating.