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Cotter Pins: What Are Cotter Pins?

How Do Cotter Pins Work?cotter pins

A cotter pin acts as a locking mechanism and can be used in many different situations. Cotter Pins are a simple to use fastener that require nothing more than a bolt with a hole in it (and a drill if the bolt does not have a hole), a pair of pliers and a castle nut.

How To Install A Cotter Pin

Installing a locking pin is a simple process:

  1. Attach the castle nut to the bolt and fasten it into place.
  2. Make a mark on the bolt or leave the castle pin installed.
  3. Using a drill and bit, drill a hole into the bolt at this point straight through the cross section of the bolt. You can make a starting point by using a metal punch.
  4. Remove any metal filings from the hole and insert the cotter pin.
  5. Using pliers, bend the ends of the cotter pin away from each other so the cotter pin cannot come out of the hole.

Pro Tip: When bending the cotter pin, try to round the points to look back in at the fastener. This will prevent them from catching on anything accidentally.

To Watch The Installation Of A Cotter Pin Check Out The Video Below:



Are Cotter Pins Necessary?

Cotter pins, while not considered necessary to every installation, may be necessary depending on the application. For example: A motorcycle front wheel is often held on by a castle nut and a cotter pin as it is subject to intense constant vibrations.

How Are Cotter Pins Measured?

Cotter pins have two measurements that matter for an installation. To choose the correct cotter pin you need to know the diameter and the length of the pin.

To determine the length of the cotter pin measure from the base of the rounded portion (“the eye”) to the end of the shorter side.

To determine the diameter of the cotter pin measure both pins together. The rounded portion is not supposed to fit through the hole so the hole should not be bigger than the eye.

cotter pin measurement diagram

Are Cotter Pins Reusable?

Cotter pins can be made from several different materials that are soft enough to bend. Typically, when deforming the pin once, the pins keep their strength and provide a strong locking mechanism that can be relied on.

As a general rule, cotter pins should not be reused as the extra bending weakens the pin. This can lead to the pin failing where a fresh pin would not resulting in a broken lock allowing the nut to spin loose.

How To Remove A Cotter Pin

Removing a cotter pin is an extremely simple task. Simply use pliers to bend the arms of the pin back straight enough to squeeze through the hole. If the pin is worn, these arms may simply snap off and then the rounded eye portion can simply be pulled out and replaced with a new one.

Why Should You Use A Cotter Pin

Cotter pins are used in many different installations. They can act as a quick locking mechanism that is easy to remove and replace but their main benefit is that they will prevent a nut from coming loose due to vibrations of any kind.

While many varieties of lock nut exist that are quick to install, they all come loose over some amount of time depending on how strong the vibrations are. A cotter pin completely removes the chance of a nut loosening unless the force acting on the nut is strong enough to shear through the pin.



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