Fastener Terminology – Threads

 In Education

Fastener Threads
Terminology surrounding fastener threads can be confusing, especially when you start getting into the Unified Thread Standard acronyms, but continuing our discussion of fastener terminology, we hope to help you identify thread styles as well as understand thread specifications and thread classes. Hopefully, this will give you a solid foundation from which to work as it relates to fastener threads.

Fastener Threads

External Thread

Thread on the outside of a cylindrical piece of stock.

Internal Thread

Thread on the inside of a cylindrical hole.

Threads Per Inch

A decimal count of the number of threads in one inch of screw thread. The term threads per inch(tpi) is used for inch fasteners.

Major Diameter

This is the diameter of an imaginary cylinder parallel with the crests of the thread; in other words, it is the distance from crest to crest for an external thread, or root to root for an internal thread.

Minor Diameter

This is the diameter of an imaginary cylinder which just touches the roots of an external thread, or the crests of an internal thread.

Thread Pitch

It is the effective diameter of the thread which passes through the thread in such a position that the widths of the thread ridges and thread grooves are equal and lies approximately half way between the major and minor diameters.


Unified National Coarse (UNC) is a thread form with a 60-degree flank angle rounded roots and flat crests. For a given diameter it has a larger thread pitch than an equivalent diameter UNF thread. The unified thread is based on inch sizes and was first standardized in 1948 unifying the Whitworth and American standard thread forms.


Unified National Extra Fine (UNEF) is a Unified thread form with a very fine (small) pitch that are typically used on instruments and parts requiring a fine adjustment.


Unified National Fine (UNF) is a thread form with a 60-degree flank angle rounded roots and flat crests. For a given diameter it has a smaller thread pitch than an equivalent diameter UNC thread.


Unified National (UN) thread form with a rounded root contour, applies only to external threads. (The UN thread form has a flat, or optionally, a rounded root contour.) The majority of fasteners with a Unified thread form have a rounded root contour i.e. are UNR threads.

Rolled Threads

Threads are cold formed by squeezing the blank between reciprocating serrated dies. This acts to increase the major diameter of the thread over and above the diameter of unthreaded shank (if any), unless an extruded blank is used.

Cut Threads

Threads are cut or chased; the unthreaded portion of shank will be equal to major
diameter of thread.

Thread Classes

1A and 1B: For work of rough commercial quality where loose fit for spin-on-assembly is desirable. 2A and 2B: The recognized standard for normal production of the great bulk of commercial bolts, nuts and screws. 3A and 3B: Used where a closed fit between mating parts for high quality work is required.

Hopefully, you find this list useful. Next time you need to impress someone at a party, tell them about the difference between UNC, UNEF, UNF, and UNR. That should really leave an impression… or maybe not. Anyway, what did we forget? What else should be included? Tell us in the comments!

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