A cotter pin is a bent wire that passes through a hole in a shaft and through the slot of a castle nut. It stops the nut from turning off. A slip pin does a similar job on a clevis or a linkage.
What it does
It locks a nut in place when the torque alone is not enough, for example on a wheel axle or brake rod.
Signs it is worn or failing
- Pin bent many times, cracked or rusted thin.
- Pin missing, or worn through.
- Nut loose: Nuts and bolts keep coming loose.
When to replace it
Fit a new pin each time the old one has been bent open and closed. A cheap new pin is better than a fatigued one.
What to match when buying
- Diameter and length to suit the hole and the nut.
- Stainless pins resist rust.
- Keep a few spare in the tool kit.
Fitting notes
- Round end forward, towards the front of the bike.
- If the slots do not line up at the right torque, tighten further. Do not loosen the nut.
- Bend one end at about 45 degrees so the pin can come out again.
- See Fit and remove cotter pins.