Thread Callouts
Form and Series
| UNC | Unified National Course |
| UNF | Unified National Fine |
| UNEF | Unified Extra Fine |
| UN | Uniform Pitch |
| UNM | Unified Miniature |
| NC | National Course |
| NF | National Fine |
| NEF | National Extra Fine |
| UNR | Unified National Round |
| NS | National Special |
| NPT | National Pipe Taper |
| BSP | British Standard Pipe |
Series
Coarse series
Used for quick assembly or disassembly of cast iron, soft metals, and plastic.
Fine Series
Used when high force is necessary for assembly. Used extensively in aerospace and automotive industries.
Extra Fine Series
Used when length of engagement is short and the application calls for high degrees of stress.
Constant Pitch Series
Used for large-diameter or high-pressure settings.
Class of Fit
Three classes of fit are established by ANSI. The classes indicate the tolerance or tightness of fit between mating threaded parts.
Class 1 – a loose fit where quick assembly is required and looseness or play between parts is acceptable.
Class 2 – a high-quality, general-purpose, commercial class of fit for bolts, nuts, and screws widely used in mass production.
Class 3 – a very high-quality threaded fastener with a close fit, used for precision tools and for high-stress and vibration applications.