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Unified screw threads -- Screw threads with diameters below ¼ in. Requirements
This part of BS 1580 specifies limits of sizes, and tolerances, for single-start, clearance-fit unified screw threads, in inch units with diameters below 
in.
This part of BS 1580 specifies unified screw threads from Number 0 (0.060 in diameter) to Number 12 (0.216 in diameter), known as the “numbered sizes” in the following standard series:
a coarse thread (UNC) series;
a fine thread (UNF) series;
for Number 12 threads, an extra fine (UNEF) 32 threads per inch diameter/pitch combination.
NOTE Unified screw threads below
in diameter for attachment purposes and for general use in the aircraft industry are specified in Annex A.
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