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Unified screw threads -- Screw threads with diameters ¼ in and larger. Requirements
This part of BS 1580 specifies limits of sizes, and tolerances, for single-start, clearance-fit unified screw threads, in inch units, with nominal diameters 
in and above, having the ISO basic profile as specified in BS ISO 68‑2:1998.
The part of BS 1580 specifies the following unified thread series as follows:
standard variable pitch series:
a coarse thread series (UNC) from 
in to 4 in nominal diameter;
a fine thread series (UNF) from 
in to 
in nominal diameter;
an extra fine thread series (UNEF) from 
in to 
in nominal diameter;
standard constant pitch series:
a 4-threads per inch series (4 UN) from 
in to 6 in nominal diameter;
a 6-threads per inch series (6 UN) from 
in to 6 in nominal diameter;
an 8-threads per inch series (8 UN) from 
in to 6 in nominal diameter;
a 12-threads per inch series (12 UN) from 
in to 6 in nominal diameter;
a 16-threads per inch series (16 UN) from 
in to 6 in nominal diameter;
a 20-threads per inch series (20 UN) from 
in to 3 in nominal diameter;
a 28-threads per inch series (28 UN) from 
in to 
in nominal diameter;
a 32-threads per inch series (32 UN) from 
in to 1 in nominal diameter;
a special thread series (UNS) with diameter/pitch combinations other than those specified for the standard series.
NOTE 1 A transition fit series and an interference fit series of UNC and UNF threads for application to screwed studs are given in BS 2693‑1 and -2, respectively.
NOTE 2 Bases of tolerances and allowances are given in Annex A, guidance for calculating limits of size is given in Annex B, information on crest forms of external threads is given in Annex C, some general symbols are given in Annex D (see 3.2), information on tolerances is given in Annex E, guidance on selection and use of the standard unified thread series is given in Annex F and information and guidance on tolerance classes is given in Annex G.
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