Cotter pins, also called split pins or cotter keys, are bent pins with a rounded head that is slid into a hole. Once inserted into the hole, the two ends are bent into different directions deforming the pin, so it cannot be easily removed from the hole. While cotter pins come in many shapes and sizes, the most common is the split pin variety.
Cotter pins are used with castle nuts to lock the nut onto a bolt and prevent it from spinning loose accidentally. They are usually made of soft metal, so they can be bent by hand or pliers to make them challenging to remove. It is common to see a cotter pin reused after being removed if it does not break during removal. This is not a good practice. The pin should be replaced with a new pin, as metal fatigue will prevent the pin from performing to standards once bent again.
*Due to materials' softness, we do not advise using cotter pins where strong shear force is a concern. In some rare cases, you may use split pins as low-tech shear pins.
Materials for Cotter Pins
Stainless Steel Cotter Pins
It is a strong material and is corrosion resistant. Stainless steel cotter pins aids in protection against corrosion and oxidation.
Brass Cotter Pins
Brass cotter pins are rust-resistant, non-magnetic and electrically conductive in nature.
Zinc Plated Steel Cotter Pins
Zinc-plated steel cotter pins are corrosion resistant and have good strength.
What are cotter pins used for?
Cotter pins are used as a locking device to hold pins or castle nuts in place. They are inserted into open holes or slots in fastener assemblies to lock the assembly in place, usually by bending the cotter pin afterwords. Refer to our video on the subject for an example of how they are used: How to install a Cotter Pin in a Castle Nut
What is the difference between a cotter pin and a split pin?
In the United States, they are known as cotter pins or cotter keys, and there is no difference between a cotter pin and a split pin, with split pin being the technical term for the fastener. In the UK, a cotter is a wedge-shaped fastener used to lock bike pedal cranks to the bike axle, which is where the confusion stems from. You may hear these fasteners called "split cotters" to help avoid confusion.
Are cotter pins one time use?
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 new pin would not result in a broken lock allowing the nut to spin loose.
What is the proper way to bend a cotter pin?
We recommend using a pair of pliers to bend the cotter pin legs around the assembly to minimize the chances of the cotter pin legs snagging on anything. You can see this in action in our video on the subject: How to install a Cotter Pin in a Castle Nut
What type of nut is used with a cotter pin?
Castle nuts, or castellated nuts, and slotted nuts are designed to be used with cotter pins which fit through the slots and a hole made in the screw to lock the assembly in place.