Introduction to Thread Pitches

For the purposes of this course, thread pitch refers to how many threads are in a 1″ length of thread. We will focus on Unified National Coarse (UNC), Unified National Fine (UNF) or sometimes called (SAE), and the 8 – Thread Series. (8UN). There are many other types of thread pitches that you will learn about later. These are the most common.

UNC-Coarse threads have fewer threads per inch and are often preferred in construction because they can be assembled rapidly and are less likely to be damaged by corrosion or handling as well as being less likely to cross thread.  Coarse threads are also more suitable for thicker corrosion resistant coatings due to the depth of the thread and the space between threads.  Coarse threads are specified on the majority of ASTM construction fastener grades.

UNF/SAE-Fine threads have more threads per inch. The threads are not as deep and are closer together than coarse threads.  This makes fine threads more susceptible to damage from corrosion, cross threading, as well as being less suitable for thicker corrosion resistant coatings.    They are, however, stronger and less likely to loosen from vibration due to more surface-to-surface contact between nut and bolt.  Fine threads are often used in the automotive industry and in machine applications.

8UN-The 8 thread series is defined by it’s use of 8 threads per inch regardless of diameter and is typically used on bolts 1” in diameter and larger.  8 thread bolts are most often used in high temperature/high pressure situations or where the engineer is looking for benefits of both UNC and UNF threads.

When describing a thread pitch here at J&E you will typically need to know two things.  First, the diameter of the material and second, the actual number of threads per inch (TPI).  The diameter may be a screw size (0-12) for small parts less than 1/4” or may be a fractional size for parts 1/4” and larger.  The number of threads per inch (TPI) will be the corresponding thread count for that diameter based on UNC, UNF, or 8UN thread counts.  On the next page you will find a chart listing each of these sizes and their corresponding thread counts.

The major thread pitches you will need to know are ¼” through 1 ½”. These are thread pitches you will see almost every day.

 You will notice that as the diameter increases the number of threads per inch (TPI) decreases in the UNC series.  This is also true for the UNF series until you reach diameters larger than 1”.  All sizes larger than 1” have a TPI count of 12 for the UNF series.  The chart also illustrates what we learned earlier about the 8UN series and sizes above 1”.

Understanding thread pitches is one of the basic, but critical, parts of learning fasteners and being able to correctly identify parts.  Almost everything we sell has a thread pitch of some kind associated with it and your understanding and knowledge of the thread pitches will impact you on a daily basis.

Fractional Equivalents for Common Numbered Tapping, Wood, and Machine Screws:
6 = 1/8
10 = 3/16
14 = 1/4
20 = 5/16
24 = 3/8

Certain fasteners (B7 Rod, Specific grades of nuts, and cap screws, etc.) Are available in
NC & NF. Plus 1″ and above in medium thread (8 pitch). inch, inch, inch, inch