Unified Coarse Threads

UNC Threads
 

Unified Fine Thread

UNF Threads

Unified Thread Standard

The Unified Thread Standard (UTS) defines a standard thread form and series, along with allowances, tolerances, and designations, for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millimetre value. The UTS is currently controlled by ASME/ANSI in the United States.

It should be noted that sizes below ¼ are given a number defined by the standard. These smaller sizes are commonly referred to by the number and pitch ie. 10-24 (UNC No.10) or 10-32 (UNF No.10)

UNF Threads

The basic profile of all UTS threads (UNC and UNF) is the same as that of ISO metric screw threads. Only the commonly used values for Dmaj and P differ between the two standards. More detail at Wikipedia Unified Threads