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How to Measure a Cotter Pin for Industrial Applications


Cotter pin assortment

The right cotter pin should have prongs that easily fit into your application's hole, a rounded top that is wider than the hole, and prongs that are long enough to be bent (if desired) once inserted. To measure a cotter pin, you will need to use calipers and a gage to ensure accurate results. First, measure the pin's length from the shortest prong from the neck of the cotter pin where it is inserted into the hole gauge. Then, measure the diameter of the shank and each prong to ensure they can be bent properly after insertion.


Cotter pins are essential components in a wide variety of industrial applications. They are small, simple wire form fasteners that keep pins or other parts in place by creating a lock when inserted through a hole. Ensuring that a cotter pin fits correctly is crucial for maintaining the stability and safety of machinery or equipment.

This guide explains how to measure a cotter pin accurately, with a focus on the steps for industrial cotter pins. We’ll start with some background on what a cotter pin is before diving into the steps you should follow in order to accurately measure your cotter pin.

What is a Cotter Pin?

The most common type of cotter pin, also known as a split pin, is a metal fastener with a split shank that can be bent around a shaft to lock components together. It typically has a flat or looped head on one end and a pronged end that splits into two tines, which can be bent after insertion to hold the pin in place. In industrial applications, cotter pins are commonly made from durable materials like stainless steel or carbon steel to withstand heavy loads and harsh environments.

In addition to the configuration above, you can find cotter pins in a variety of other styles, including hump, hammerlock, self-locking, and clinch pins. These cotter pin varieties can have customized end types, such as square ends or chisel point ends, depending on the industry and application.

Why Accurate Measurement is Important

For cotter pins to function correctly, they must match the specifications of the application. An undersized cotter pin may not secure components tightly, leading to potential malfunction, while an oversized pin might not fit into the designated hole. Precise measurement is especially important in industries where machinery is under constant stress, as in the automotive, heavy machinery, and aerospace industries.

In addition to measuring a cotter pin, you should always visually inspect your part. For instance, you should be able to see that the legs of the cotter pin are straight with very minimal misalignment or bowing and the cut should be clean with no burrs. Gaps between the prongs should be within .015” with contact at the ends.

Tools Needed for Measuring Cotter Pins

To accurately measure a cotter pin, you will need a few common tools. Before starting, gather the following:

  • Calipers, preferably digital calipers for increased precision
  • A gage to determine specific hole diameter
  • Ruler
  • Micrometer (optional, for high-precision measurements)
  • Pen and paper or computer, or phone to record measurements

Steps to Measure a Cotter Pin

Cotter pin measurement diagram
The image above shows an extended prong, chisel point cotter pin. In this image, A is the total shank diameter, B is the wire width, C is the head diameter, L is the prong length, and D is the extended prong.

To measure a cotter pin, you will need to determine the size of four key dimensions: total length, shank diameter, head diameter, and prong length. Follow the steps below to find each dimension.

Measure the Length of the Cotter Pin

When measuring the cotter pin’s length, first insert the cotter pin into the diameter hole gage. Then, measure the length of the shortest prong from the neck of the cotter pin where it is inserted into the gauge.

  • Step 1: Place cotter pin in the gage, ensuring easy fit.
  • Step 2: Measure the length of the short leg by placing your calipers flat on the plane of the gage, ensuring to stay parallel to the leg, and at the longest point of the short leg.
  • Step 3: Record the total length. The length is typically expressed in millimeters or inches, depending on the standard in your industry.

Measure the Diameter of the Shank

  • Step 4: Use a pair of calipers to measure the diameter of the pin’s shank. The shank diameter is the thickness of the cotter pin at the end of the pin. There should be no gap in this area. Ideally you would want to measure the diameter at multiple points along the shank to ensure proper fit. It is important to measure at the bottom of the pin to ensure the part will go into the hole size desired.
  • Step 5: For the best results, measure the diameter at multiple positions along the shank to ensure the proper fit. Make sure at least one of these measurement is at the bottom of the pin. This measurement is crucial as it will help you ensure that the cotter pin will go into the hole size you desire. The shank diameter needs to be smaller than the hole diameter.

How to Measure Round Wire that is Rolled to Half Round

When using round wire that is rolled to half round, measure the diameter of the half round by dividing the overall diameter by two. In this case, the cotter wire comes in as round wire that then goes through a wire roller. The wire roller gives it a half rount with the flat on one side. This is because wire rollers have two drums. The bottom has a flat and the top has a radius with a certain depth and width depending on the wire size needed. As the wire goes through the roller, it is smashed down to form the wire, making it necessary to measure the half diameter of the wire after it has gone through the roller to ensure it does not fall above or below the required tolerances to keep the overall diameter of the pin in spec. For example, for 1/8” diameter cotter pins, you would use 0.125” wire, which is then smashed down to 0.058” – 0.062”.

Measure the Head Diameter (Outside Diamete)

  • Step 7: Place your calipers or ruler along the widest section of the cotter pin’s head and measure the diameter. This measurement should be larger than the diameter of the hole the cotter pin will be inserted into.

Measure the Prong Length

  • Step 8: For specific applications, you might need to measure each tine or prong to ensure they can be bent properly after insertion. This is often the case when the prongs need to be bent over an axle or around a shaft.
  • Step 9: Use a ruler or calipers to measure from the point where the tines split to the end of each prong. Since cotter pins can vary slightly, measure both prongs and record each length separately. Using the depth gage on your calipers is the best way to get an accurate measurement.
  • Step 10: Note any significant discrepancies between prongs. Depending on the type of cotter pin, prongs can be equal, or one may be longer than the other. Prongs will need to extend beyond the depth of the hole they are inserted into so they can be bent and provide appropriate fastening.

How to Interpret Your Measurements

After measuring, you need to confirm compatibility with the hole or shaft. Compare your measurements with the specifications in your equipment’s technical manual or guidelines provided by your company. Here are some general points to consider:

  • Tolerance Levels: Ensure that the diameter matches or falls within the tolerance levels specified. Cotter pins often have some leeway in diameter size but should not vary more than 5-10% from the required width.
  • Length Precision: The pin length should be close to the specified range, but some flexibility is acceptable if the prongs are long enough to be bent securely.

Important Tips for Cotter Pin Measurement and Selection

Check Standards: Many industries follow specific standards, such as those set by the American Society of Mechanical Engineers (ASME) or International Organization for Standardization (ISO). Make sure your cotter pins meet any necessary standards, especially in safety-critical applications.

Check Ductility: Prongs should be able to be bent 180° without fracturing.

Use Quality Cotter Pins: High-quality pins are more resistant to deformation and wear, reducing the likelihood of equipment failure. Our Western Wire location has manufactured high quality industrial cotter pins for nearly 100 years. Shop our selection online or contact us for a custom cotter pin quote.

Consider Material: When measuring a replacement cotter pin, confirm the material of the original pin. Industrial cotter pins are commonly stainless steel, carbon steel, or brass, each suited to different environments. Selecting a replacement cotter pin made from a different material may affect end results.

More Information on Cotter Pins

Accurately measuring your cotter pin and the components it will be used to secure will help you purchase the right part for your application. For more information on cotter pins, visit our cotter pin page or contact us. We have in-house engineering and manufacturing experts who can help answer any questions you may have.

Cotter Pin Products Shop In-Stock Cotter Pins

Explore More Cotter Pin Resources

Our Western Wire location has been in business for over a hundred years and has in depth experience designing, measuring, and meanufacturing cotter pins. Explore additional resources from Western Wire below or, if you have any questions, reach out to their experts directly. You can contact Western Wire here, request a quote to start your project, or use our ask an expert form to request one-on-one help from one our engineers.

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