A cottar pin is a split wire fastener used to connect two pieces of metal. The metal parts are joined with one end containing a hole for the cottar pin. The pin is inserted though the hole to fasten the parts together. Cotter pins are commonly found on the end of an axle for a walk-behind lawnmower. The wheels slip over the axles, which have drilled holes in the ends for cottar pins to hold the wheels in place.
Connect the parts to be secured by a cottar pin. For example, a wheel slips onto the end of an axle, leaving the cottar pin hole exposed at the end of the axle.
Insert the two tines of the cottar pin through the hole until the eye of the cottar pin is flush against the hole.
Pry apart the two tines at the end of the cottar pin using the needle-nose pliers. Bend each tine upward so that the pin cannot pull out on its own.
Make sure the cottar pin is not obstructing the movement of the two parts.