Unified National Standard (UNS) replaced American National Standard (ANS)
- UN replaced N
- UNS replaced NS
- UNC replaced NC
- UNF replaced NF
- UNEF replaced NEF
- 1A, 1B replaced Class 1
- 2A, 2B replaced Class 2
- 3A, 3B replaced Class 3
- Class 4 obsolete
- Class 5 used for interference threads
Classes 1A and 1B
- This class has the loosest fit.
Classes 2A and 2B
- This is the most common thread class and used for commercial products. This class is the typical fit.
- This class threads are loose enough to allow for expansion due to high temperatures or even the insertion of a lubricant into the threads. The G is for coated threads so they will not exceed the thread tolerance.
Classes 3A and 3B
- This is the most precise fitting thread, a very close and tight fit.