Crank Cotter Pin | 9 mm Cotter Pin for Crankset

Guide to cotter pins – choosing, measuring, and installing

Cotter pins are used on traditional cottered cranksets, which are mainly found on older and classic bikes. They come in many different sizes, and the diameter can vary considerably – on vintage bikes, you may find cotter pins from around 7 mm and up.

Available sizes include 7.0, 8.0, 9.0, 9.05, 9.30, and 9.50 mm, among others.

How to find the right size

The safest method is to measure the old cotter pin with a caliper.

Measure the diameter of the round part of the cotter pin, the part that passes through the crank arm. Do not measure the tapered wedge face.

If the old cotter pin is badly worn or missing, you can measure the hole in the crank arm, but be aware that old holes may be worn or may have been modified previously.

The cotter pin often needs adjusting

With traditional cottered cranksets, a new cotter pin is not necessarily intended to fit perfectly without adjustment.

The cotter pin has a flat side and an angled/tapered side. When it is pressed through the crank arm and around the crank spindle, the position of the wedge determines how far the crank arm sits onto the spindle.

You may therefore need to file a very small amount off the flat side of the cotter pin so the crank arm sits in the correct position.

The goal is for the two crank arms to be exactly 180° apart.

Important when adjusting

• Remove only a very small amount of material at a time.
• Test the cotter pin several times as you go.
• Check the position of the crank arms after each adjustment.
• Make sure the cotter pin is correctly seated and has full contact with the crank spindle.
• The cotter pin must not be so loose that the crank arm can move on the spindle.
• Avoid filing the round part of the cotter pin to make it smaller. The diameter must match the hole.

Once the fit is correct, tighten the nut so the crank arm is secure and has no play.

Diameter is not the only thing that needs to match

Even if two cotter pins have the same diameter, they are not necessarily identical. Length, the shape of the wedge face, and the position of the tapered section can vary.

That is why the old cotter pin is always the best reference, if available.

Vintage bikes may also have different standards and manufacturer-specific designs. Therefore, it is not always possible to determine the correct cotter pin based solely on the bike's brand or year.