In almost all thread series, the number of threads per inch decreases as the diameter increases. It became obvious to engineers using very large bolts on bridges and similar structures that this practice makes no sense for very large bolts. So for large bolts three series were defined in which all diameters have the same pitch; they are simply lists of standard diameters. All the bolts in the 8N series had 8 threads per inch, all in the 12N series had 12 threads per inch (used in boilers and for thin nuts on shafts), and in 16N, sixteen threads per inch (used where precise positioning of a big nut is required e.g., for bearing adjusting nuts).
The N constant-pitch series were later replaced by the constant-pitch Unified Thread series, which have the Unified thread form and are designated 4-UN, 8-UN, and so on. For external threads only, the UNR thread form may be used.
Sometimes, for a particular diameter, the constant-pitch happens to be the same as the pitch in another Unified Thread series, in which case the fasteners are identical. In these cases the table gives the abbreviation for the name of the other series.
The pink cell background indicates non-preferred diameter values. The green columns are simply an aid to orientation.
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Last revised: 3 April 2006.