All you need to know about UN thread gauges


The UN thread gauges measure screw threads (UTS) using the Unified Thread Standard as its guide. Screw thread allowances, tolerances, and designations are included in this standard thread form and series, widely used in the United States and Canada. When inspecting the dimensions of threaded products, the unified system includes a list of the characteristics that must be checked and the gauge used to check them.

These countries’ threaded fasteners are all measured according to the UTS standard. UTS threads have a very similar profile to the ISO metric screw thread profile. Because of this, the UTS thread’s characteristic dimensions (outer diameter and pitch) were chosen as fractions of an inch rather than metric values. In the United States, ASME/ANSI currently controls the UTS.

Examples of Unified Threads

Uniform Pitch – UN

Standard thread counts of 4, 6, 8, 12, 16, 20, 28, and 32 are common.

United Coarseness

Low tensile strength materials are used for the most common fastener thread type, general purpose inch gauging. The threads are deeper than UNF because of their coarse pitch, making them easier to install without cross-threading.

The external thread has a higher tensile strength than coarse thread because of the smaller thread depth compared to UNC. It also has a tighter tolerance and a finder tension adjustment. The aerospace industry is a major user of this product.

The UNEF stands for Unified Extra Fine. Compared to UNF, it is used in applications like tapped holes in thin wall material and thin-walled pipes.

Commonly used threads with the outer thread having a larger root radius than those of UNC or UNF are known as UNJC & UNJF. Because of this, the tensile area is larger, and the stress concentration is reduced.

TPI (Thread per Inch)

The number of threads per inch that are present along the length of a thread is referred to as the “thread pitch” in unified threads. Thread pitch in Metric thread is more commonly used than thread per inch in Unified thread.

Threading Standardization

‘A’ and ‘B’ are the classes for the Unified Thread. Threshold gauges A and B have the same letter designation. Consequently, tolerance classes 1A and 2A apply to Thread Ring Gauge, and tolerance classes 1B, 2B, and 3B apply to internal threads. Two of the most common are Class 2A and Class 2B, respectively.

  • There is a lower risk of cross-threading when using UNC threads for component assembly.
  • Compared to UNC, UNF thread gauges have a greater tensile stress area, making it stronger under tension.
  • Threading in thinner walls is easier with UNF Thread because of its shorter threat depth.
  • WHEN FASTENING TO LOWER-STRENGTH MATERIALS, using UNC thread when fastening to lower-strength materials reduces the risk of thread stripping.
  • Due to the lower stress concentration at the thread root radius, it is more resistant to fatigue.
  • Compared to other threads, the UNF Thread has a shorter engagement length, thus providing greater strength.
  • Using UNF thread makes tapping harder materials much easier.