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Constant Pitch Series
 
Constant Pitch Series, The constant pitch series, UN, have 4, 6, 8, 12, 16, 20, 28, and 32 threads per inch. These are available for use when the Coarse, Fine, Extra-Fine Series do not meet the design requirements. Within these series, preference should be given to the 8, 12, and 16 thread series.
 
The 8 Thread Series, 8-UN is used for large diameters(greater than 1 inch) with a coarse thread. It was originally developed for use in high pressure joint bolts and nuts.
 
The 12 Thread Series, 12-UN is used for large diameters with a medium pitch thread. It was originally intended for use pressure vessels, such as boilers. It is currently used as a fine pitch series extension for diameters larger than 1½ inches.
 
The 16 Thread Series, 16-UN is used for large diameters with fine pitch threads. It is used for adjusting collars and retaining nuts and can be considered as an extension of the extra fine thread series for diameters larger than 111/16 inches.
 
The 4-UN, 6-UN, 20-UN, 28-UN and 32-UN Thread Series are used for those applications where the Standard Coarse, Fine or Extra Fine Series are not not a good choice.