UNC Thread size chart

Out of the two Unified Thread Standards (UTS), UNC threads are the most commonly used, the other being UNF (United National Fine Threads). UNC threads have specific characteristics that make them more suitable for the applications they are used for. Some such features are as follows.

Unified Coarse Threads are less likely to cross-thread between themselves cue to their texture.

UNC threads enable faster assembly and disassembly of fittings. They are used in mass production due to this feature.

UNC threads enable a thicker plating and coating of fittings as they  have large thread allowances.

When compared to UNF threads (fine threads), UNC threads have a fewer number of threads per distance.

UNC threads are subjected to a classification system which helps to make it easy to manufacture them. The thread class indicates the range of pitch diameter for any specific range of thread. These classes are specified according to their tolerances and are called the United Screw Thread Standard Series. UNC can be found in five different classes. They are as follows:

1. Class 1 UNC threads are used for applications which require a liberal tolerance is required in order to ensure an easy assembly. This is common to instances where slightly nicked threads are used. They have a loosely fitting set of threads that are used in easy assembly points or in a dirty environment.
2. Class 2 UNC threads are the types of UNC threads are the most commonly used class for general-purpose applications. These UNC threads are designed and manufactured to maximise the strength in relation to machine practice.
3. Class 3 UNC threads are used in applications where it is important to use close fit and accurate thread elements.
4. Class 4 UNC threads are manufactured to use in a closer fit than a Class 3 UNC thread.
5. Class 5 UNC threads are designed nd manufactured to be used on item,s like spring shackles in automobiles. Therefore, class 5 UNC threads are not manufactured with loose threads,

UNC threads can be identified as either internal or external threads. The letter suffix along with their class indicates whether they are internal or external threads. The letter suffix of either ‘A’ or ‘B’ is used for this purpose, where ‘A’ indicates external threads, meanwhile ‘B’ indicates internal threads. Thereby, classes 1A, 2A, and 3A indicate external threads, while Classes 1B, 2B, and 3B apply to internal threads.

The thread pitch is the distance between the threads that is indicated in millimetres. Usually, smaller fasteners have finer threads, which means they have a relatively lower thread pitch. The typical designation of a UNC thread is ¼” x 20 Unified National Coarse Thread.