Crank Cotter Pin | 8mm Cotter Pin for Crankset

Guide to cotter pins – selection, fitting and installation

Cotter pins are used on traditional cotter-pin cranksets, which are found mainly on older and classic bicycles. They come in many different sizes, and on vintage bicycles you may find cotter pins from around 7 mm and up.

How to find the right size

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

Measure the diameter of the round part of the cotter pin – not the tapered part. If the old cotter pin is missing, you can measure the hole in the crank arm, but be aware that old holes may be worn or may have been modified in the past.

Fitting the cotter pin

A cotter pin does not necessarily have to fit perfectly without adjustment. On traditional cotter-pin cranksets, it is normal to adjust the flat side of the cotter pin slightly so that the crank arm sits in the correct position on the bottom bracket spindle.

The most important thing is that the two crank arms are positioned exactly 180° opposite each other after installation.

Remove only a very small amount of material at a time with a file, and check the fit as you go.

Important: Do not try to make the round part of the cotter pin smaller. Its diameter must fit the hole in the crank arm correctly.

Installation

Once the crank arm is correctly positioned on the bottom bracket spindle, insert the cotter pin through the holes in the crank arm and bottom bracket spindle from the correct side.

A professional workshop uses a cotter pin press. This is the best and most controlled way to press the cotter pin into place correctly while minimizing the risk of damaging the crank arm, bottom bracket spindle or threads.

However, a hobby mechanic who only needs to install a cotter pin occasionally does not necessarily need to invest in such a press. The cotter pin can be installed carefully without special tools, but it is important to work in a controlled manner and avoid damaging the threads or crank arm.

Do not use the nut to pull a cotter pin into place if it does not already fit correctly. The cotter pin must first have the correct fit and position.

Once the cotter pin is correctly seated, fit the nut and tighten it securely.

After installation, check that the crank arm is secure and has no play.

Keep in mind

Cotter pins with the same diameter are not necessarily identical. Length, the shape of the tapered face and the position of the tapered section can vary.

Vintage bicycles may also use different standards and manufacturer-specific designs. Diameter alone is therefore not always enough to determine whether a cotter pin will fit.

If in doubt, always compare it with the old cotter pin and measure the diameter and length before ordering.