единый национальный крупной резьбы(The common V-thread standards (ISO 261 and Unified Thread Standard) include a coarse pitch and a fine pitch for each major diameter. For example, 1⁄2-13 belongs to the UNC series (Unified National Coarse) and 1⁄2-20 belongs to the UNF series (Unified National Fine). ... The term coarse here does not mean lower quality, nor does the term fine imply higher quality. <...> They simply refer to the size of the threads relative to the screw diameter.
wikipedia.orgYuV)
