| UNC |
Unified National Coarse / Unified Coarse |
Common general-purpose inch thread with a coarser pitch. Often used where assembly is frequent or material engagement is more forgiving. |
| UNF |
Unified National Fine / Unified Fine |
Inch thread with a finer pitch than UNC. Often used where finer adjustment, stronger engagement, or limited thread length matters. |
| UNEF |
Unified National Extra Fine / Unified Extra Fine |
Extra-fine inch thread used for more specialized applications where very fine pitch is required. |
| UN |
Unified Thread Series |
General unified inch thread designation. Often appears with a specific threads-per-inch value when the pitch is not one of the standard UNC/UNF/UNEF series. |
| UNS |
Unified Special |
A special unified inch thread that does not fall into the standard coarse, fine, or extra-fine series. Should be clearly defined on the print. |
| UNJ |
Unified Thread with Controlled Root Radius |
Aerospace-oriented thread form with a controlled root radius, especially important for fatigue-sensitive applications. SAE AS8879 specifies the UNJ inch profile and controlled root radius requirements. |
| UNJC |
Unified National Coarse, J Profile |
Coarse-pitch UNJ thread form. Similar application idea to UNC, but with the UNJ controlled-root profile. |
| UNJF |
Unified National Fine, J Profile |
Fine-pitch UNJ thread form. Commonly seen in aerospace-style drawings where fatigue performance and controlled thread form matter. |
| UNS / UNJF Special |
Special Unified or Special UNJ Thread |
Non-standard thread combinations that require careful review because standard gages or taps may not be on hand. |
| M |
ISO Metric Thread |
Metric thread designation. Example: M6 x 1.0. ISO 262 covers selected metric screw thread sizes for screws, bolts, and nuts. |
| NPT |
National Pipe Taper |
General-purpose tapered pipe thread. ASME B1.20.1 covers NPT and related inch pipe threads, including dimensions and gaging. |
| NPTF |
National Pipe Taper Fuel / Dryseal |
Tapered dryseal pipe thread intended to create a tighter seal through thread form interference. Often used in hydraulic, fuel, or leak-sensitive applications. |
| NPSM |
National Pipe Straight Mechanical |
Straight pipe thread used for mechanical joints, not the same sealing behavior as NPT. ASME B1.20.1 includes NPSM among covered pipe thread types. |
| NPSC |
National Pipe Straight Coupling |
Straight pipe thread used for couplings. Also part of the ASME B1.20.1 pipe-thread family. |
| BSPP / G |
British Standard Pipe Parallel |
Parallel pipe thread commonly seen on international or fluid-power components. Usually not interchangeable with NPT. |
| BSPT / R |
British Standard Pipe Taper |
Tapered British pipe thread. Common on international equipment and fluid systems. |
| STI |
Screw Thread Insert |
Thread callout for holes intended to accept helical or solid thread inserts. The tapped hole is not the same size as the final screw thread. |
