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How to Install a Cotter Pin on a Castle Nut

Importance of Correct Cotter Pin Installation

Cotter pins are used with a variety of steering and suspension parts like ball joints, control arms and tie rods. Proper installation of the cotter pin aids the retention of fasteners while in service. An improperly installed cotter pin could be an indication of other problems that could lead to stud breakage or other damage.

Proper Technique

Prior to tightening, fasteners ensure that studs have a secure fit with their receiving hole. If there is any looseness, it could be an indication of damage to the receiving hole and could lead to stud breakage. After installing the stud, castle nuts must be torqued to the manufacturer recommended specification. Always use a torque wrench; don’t use an impact gun. If a washer was provided, it should be placed under the nut before installation. Failing to use the washer can cause the cotter pin hole to be below the castle nut (as shown below).


When correctly installed, the cotter pin hole should align with the slots (castellations) in the castle nut. If necessary, the nut may be tightened slightly to improve the alignment; never back the nut off. If the cotter hole is too high or too low, it could be an indication that the stud has a poor fit to the tapered hole or that the part is not correct for the application. After aligning the slots in the nut with the cotter hole, insert the cotter pin completely. Bend the legs of the cotter pin back to prevent it from slipping out. The cotter pin provides additional assurance that the nut will not back off while in service.

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The content in this article is for informational purposes only. You should consult with a certified technician or mechanic if you have questions relating to any of the topics covered herein. Tenneco will not be liable for any loss or damage caused by your reliance on any content.

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