Unified National Fine (UNF)

Unified National Fine (UNF) is the fine-pitch thread series within the Unified Thread Standard (UTS) for inch-based fasteners. Like other UTS designations, a UNF callout lists the nominal diameter and threads per inch (TPI), such as 1/4-28 UNF or 3/8-24 UNF. Compared to UNC, UNF uses a higher TPI for the same diameter, meaning the threads are more closely spaced.

UNF threads are often chosen when you need more precise adjustment, strong clamping performance, or improved resistance to loosening under vibration. Because the thread helix angle is smaller, fine threads can better convert torque into tension in many assemblies and can provide greater thread engagement length over a given distance. The tradeoff is that UNF threads are generally more sensitive to damage and cross-threading and may be less forgiving in dirty conditions or softer materials. They’re common in automotive, aerospace, machinery, and other applications where consistent preload and compact, high-performance fastening are important.