What does UNC/UNF mean?

The American Unified National Thread Series regulates thread series according to ASME B1.1, this standard includes shape, series, class, dimensions, tolerance and designations. UNC and UNF threads are the most common in the USA and Canada.

The American Unified National thread according to ASME B 1.1 has a flank angle of 60°. The thread profile and possible applications are comparable to the metric ISO thread according to DIN 13.  

UNC describes the coarse thread, comparable to the metric standard thread. UNF, on the other hand, describes the fine thread. UN threads have constant pitches per inch and are therefore fixed.

The designation of the thread consists of the nominal size in inches, followed by the pitch, while the letter C or F designates the thread type, i.e. coarse or fine thread. Diameters less than 1/4 inch are designated by a number defined in the standard. E.g. No. 8 - 36 UNF threads. The gradient is in inches.