Cars Simplified: Everything Automotive Explained

Cotter Pins

A Stainless Steel Cotter Pin

Cotter pins are used to hold components in place, such as castle nuts on a bolt with a pin hole, or a brake fluid reservoir to a brake master cylinder. They are commonly made out of metal and usually aren't re-used when a service requires them to be pulled out, because they are very inexpensive and wear out.

Cotter pins are installed by inserting them in a hole, followed by bending the ends apart opposite each other so that they resist pulling back through the hole they were inserted into. The ends can be bent in the same direction if limitations force this, but opposite directions are more reliable.

How to Remove a Cotter Pin

Cotter pins are designed to slide into a hole while they are straight, and then be bent so they are no longer straight and won't slide back out, so using a tool to get the ends straight before pulling on the eyelet side aids removal. Pulling on the eyelet side can force the tips to straighten out enough to slide through if the tips are straightened enough to not resist movement in that direction. Typically needle nose pliers are used to grab the pin.