There is a wide array of threaded fastener types, including:
UNC threads are the most common general fastener thread. Their fit is deeper and more generic than that of a fine thread, allowing for easy removal. Generally, they have a higher tolerance for manufacturing and plating, and do not need cross threading to assemble.
UNF threads have better torque-locking and load-carrying ability than UNC threads because of their larger minor diameter. Because of their more specific fit, they have tighter tolerances, finer tension adjustment, and can carry heavier loads. They are most commonly found in the aerospace industry.
UNC threads are the most common general fastener thread. Their fit is deeper and more generic than that of a fine thread, allowing for easy removal. Generally, they have a higher tolerance for manufacturing and plating, and do not need cross threading to assemble.
UNEF threads are finer than UNF threads; they are used in applications with tapped holes in hard material, thin threaded walls, and tapped holes in thin material. As with UNF threads, UNEF threads are common in the aerospace industry.
The Unified thread angle is 60° and has flattened crests (Whitworth crests are rounded). From 1/4 in up to 1 1/2 in, thread pitch is the same in both systems except that the thread pitch for the 1/2 in bolt is 12threads per inch (tpi) in BSW versus 13 tpi in the UNC.
The ISO metric screw threads are the world-wide most commonly used type of general-purpose screwthread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.
There are three classes of thread fit:
Designations for each class and the type of thread(inside or outside) for Unified and Metric are given below.
| UNIFIED | METRIC |
|---|
| CLASS | EXTERNAL THREAD | INTERNAL THREAD | EXTERNAL THREAD | INTERNAL THREAD |
| Loose | 1A | 1B | 8g | 7h |
| Standard | 2A | 2B | 6g | 6h |
| Close | 3A | 3B | 4g | 5h |