N vs. UN
NC vs. UNC
NF vs. UNF
NEF vs. UNEF
NS vs. UNS

SUMMARY
The American National Standard has been obsolete since 1949 and replaced with the Unified National Standard. In all cases, the threads made to the Unified National Standard are designed to screw together with the obsolete American National Standard. Without exception, drawings should be updated to reflect the current standard. The class-of-fit requirements for the obsolete American National Standard can be translated to the current Unified National Standard. As always, obtain approval from your customer.

HISTORY
The American National Standard was replaced with the Unified National Standard for two reasons. The first reason was to provide interchangeability with Canada and United Kingdom. The second reason was to correct certain thread production difficulties.

Thread makers were told to translate the obsolete American National Standard thread requirements on existing drawings to the Unified National Standard using the comparable class-of-fit. There was resistance to change because of the existing inventory of gages. Thread makers were told to use their existing gages until they needed to be replaced and then replace them with the Unified National Standard gages.

TRANSLATION
N changes to UN
NS changes to UNS
NC changes to UNC
NF changes to UNF
NEF changes to UNEF
Class 1 changes to 1A for external threads or 1B for internal threads
Class 2 changes to 2A for external threads or 2B for internal threads
Class 3 changes to 3A for external threads or 3B for internal threads
Class 4 obsolete
Class 5 is still used for interference threads

SUMMARY OF DIFFERENCES
Several changes were made that were specific to nomenclature. Minor changes were made to the general thread form of the end product conform to manufacturing realities. Some benign changes were made relating to the major and minor diameters. Changes were also made to pitch diameters to remove tolerance issues which made the threads nearly impossible to manufacture and gage. Under the obsolete American National Standard the product tolerances were practically absorbed by the combined tool and gage tolerances, leaving little working tolerance in manufacture.

COMMENTS
Refer comments/corrections to gageguy@gagecrib.com.
Revised 3/21/2008