Understanding Metric and Inch Thread Sizes in Fasteners

5 min read

It can be challenging to sort through the differences between Inch and Metric screw designations. Anyone that has taken a trip to the local hardware store searching for a screw to fit an existing nut or bolt fastener knows this to be true.

The problem of determining how screws fit together into holes and designing a suitable screw/hole or screw nut fit can be complex without adequate background knowledge. To uncover the solution, let’s dive into the two specifications without further delay.

Inch Screws Specifications Explained

To grasp the Inch Screw size designations, it’s helpful to show a screw size specification and then break down every single part in the specification to show what it is and what it means. For example, let’s start with a designation that looks like the following:

#4-40 UNC-3A x 0.5

This designation will typically be prominently featured on the box or machine design figures, so you don’t have to look too hard to find it.

#4-40 UNC-3A x 0.5

The first number is the thread diameter. Here it represents a No. 4-sized screw. No. 4-sized screws have a diameter of approximately 0.120”, which you can quickly look up on a machine screw size chart. The number can also be listed as a fraction, such as 3/16, wherein the fraction represents the diameter of the screw in inches.

#4-40 UNC-3A x 0.5

The second number represents an essential descriptor of the threads present on the screw. This number represents the thread pitch or the term for the number of threads per inch of screw length. This number determines how coarse (e.g., fewer threads per inch) or fine (e.g., more threads per inch) the thread pitch is. Screws must be used in holes with matching pitches, or the parts will not fit together.

#4-40 UNC-3A x 0.5.

The third item of interest is the thread specification, usually denoted by a three-letter callout, most commonly UNC or UNF. These are the shorthand notations for the thread standards, in this case, Unified National Coarse (UNC) and Unified National Fine (UNF).  These thread standards shed more critical information on the thread and tell you if the thread is coarse or fine. In general, UNC is the most common thread callout, while UNF threads, being finer, are used for more specialized applications such as vibration environments. In some instances, you may see the thread callouts UNJC or UNJF, which are the same as their more common counterparts, with the exception being that the “J” threads have slightly larger root radiuses, making them stronger and more suited for applications that demand such strength.

#4-40 UNC-3A x 0.5

The fourth number is the designation which shows what the tolerances class is. Essentially the “size 1” screws have the most generous tolerance and are used for loose fits, while the “size 3” screws have the tightest fit. The A and B denote whether threads are “external” for a screw” or “internal” for a hole. In this case, a screw thread is an external thread, while a nut or tapped hole is always an internal thread. The important note is that the tolerance classes for holes or nuts should match the corresponding screw class. The takeaway message for this number is simply that the value provides additional information to help you select the screw that perfectly fits your application’s needs.

#4-40 UNC-3A x 0.5

The final number is the length of the screw. This number can be written as either a decimal or a fraction.

The only other exciting thing to note is that sometimes the term “LH” in the screw callout denotes that the screw is a Left-Hand fastener.

Metric Screw Specifications Explained

With the background on Inch screw designations in hand, the Metric designations will be much easier, and the explanation will be much simpler.

Metric screw sizes follow a similar nomenclature and display the same information as the Inch thread specifications. However, a few key distinctions and differentiations are essential to review.

The first big giveaway that you’re dealing with a metric specification is the “M” label on the front of the specification. Not surprisingly, this standard for “Metric” should immediately tip you off that you’re looking at a metric screw. For example, a metric screw specification looks like this:

M10 – 1.0 x 20

Once you’ve established the screw is metric, the following few numbers are easily explained.

M10 – 1.0 x 20

The first number is the nominal diameter of the screw, listed in millimeters.

M10 – 1.0 x 20

The second number is subsequently the thread pitch, the same as the Inch specification. Here it is measured as the distance between threads, denoted as “1.0”.

M10 – 1.0 x 20

The third and final number is the fastener length, listed in millimeters, here indicated as 20 millimeters. This placement at the end is the same as the Inch thread standard which lists the thread length at the end.

There you have it! The differences between metric and Inch screw designations are not intimidating at all. A better understanding of these thread standards will enable you to incorporate both into your machine designs. To this end, MISUMI carries a wide range of fasteners across many specifications to meet your needs, explore them here.

About the Author

Carlicia Layosa

Carlicia is the Marketing Automation Manager at MISUMI. She holds a bachelor's degree in Mechanical Engineering and a master's degree in Energy Engineering from the University of Illinois at Chicago. She is a Certified SOLIDWORKS Associate, Marketo Certified Expert, and is passionate about education and training.

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