What is a Cotter Pin?

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  • Written By: Malcolm Tatum
  • Edited By: Bronwyn Harris
  • Last Modified Date: 14 January 2020
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Cotter pins are simple devices that are used in many different types of machinery. The exact configuration and use of the pin will vary from one location to another. For the most part, the pin is utilized to provide a secure connection between two components that is not loosened while the machinery is on operation. Just about any type of machinery that employs the use of nuts and bolts in the design is also likely to require the presence of a cotter pin.

In the United States, the cotter pin is typically a slender pin that is split in the middle. Stainless steel is often the metal of choice for these devices. The head of the pin is normally larger in diameter than the body of the pin, making it easy to slide the device into place through a nut or bolt. Once in place, the two tines that make up the body of the pin can be bent around the nut or bolt to help secure the components in position securely. In this application, the cotter pin performs a function similar to a rivet, but can be used when a rivet is not a practical solution.


The cotter pin in manufactured in a variety of sizes. This makes it possible to utilize this type of fastening device in both small and large applications, ranging from simple tools and appliances used at home to large pieces of machinery that are used in textile plants, electronic assembly plants, and other large scale operations. Relatively inexpensive, cotter pins are often purchased in lots of a hundred or more, depending on the size and type of material used to create the pins.

There are a number of different types of cotter pins in use today. Among the cotter pin assortment utilized in different settings is the R-clip pin and the bowtie pin. There is even a round or circular cotter pin that is known as the circle cotter. Each of these perform the same basic function of securely connecting two components, but are utilized in different settings, based on the type of machinery involved and the size of the components that need to be connected.

In the United Kingdom, a cotter pin may also have a slightly larger body that allows the device to be firmly wedged through holes in two components to hold them in position. This is slightly different from the thinner body found with most cotter style pins in the United States, but is especially helpful where there is no real desire to allow any type of friction or slippage between the connected components. As more corporations have become international in scope, the variety of pins that are part of the cotter pin family has sometimes led to confusions, based on different understandings of what constitutes this type of pin in different cultures and settings.


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