The coarse and fine thread series of the Unified Inch thread system.
Unified National Coarse (UNC) and Unified National Fine (UNF) are the two series in general use. A given nominal diameter has one pitch in each: 1/4-20 is UNC and 1/4-28 is UNF; #10-24 is UNC and #10-32 is UNF.
UNEF is a third, finer series used in aerospace and instrumentation. UNJ is a controlled-root-radius variant with higher fatigue strength, used in aerospace fasteners and not interchangeable with UN.
Assuming UNC and UNF of the same nominal size are interchangeable. They have different pitches and will not assemble.
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