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What is the difference between UNF and UNC threads?

Weimi Hardware Tech. Company | Updated: Jan 05, 2018
Source: Rob Lion & Loring Chien, search via Bing

What is the difference between UNF and UNC threads?

UNF and UNC are the two main categories of the Unified Thread Standard used in the United States. For a given nominal outside diameter of a screw, there are Fine and Coarse thread densities (thread pitches), measured in Threads Per Inch (TPI). For example, a 1/4″ nominal screw diameter could have a UNC 1/4″-20 thread, or a UNF 1/4″-28 thread. These have, respectively, 20 threads per inch and 28 threads per inch.

For larger diameters, there is also a UNEF (Extra Fine) pitch option, though it is not nearly as commonly used as the other two. The ISO metric screw thread standard has similar Coarse and Fine thread pitches, although they are specified as the inverse millimeters [per thread] rather than Threads Per Inch.

There are a variety of tradeoffs in engineering design that motivate the selection of fine or coarse screw thread options, which depend on the specific application and are too complex to get into here.


Thread pitch is the number of threads per unit distance in threaded rod or bolts or machine screws.

UNC and UNF are specific AN SI standard pitch series for different diameters.

For example 1/4 inch rod the UNC is 20 tpi and UNF is 28 tpi the former being the coarse series and the latter being the fine series.

Threads per inch.


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