Unified National Standard

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

Tolerance Classes

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.

Classes 2AG

• 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.