1. Nuts - Both the kingpin and axle nuts have a nylon locking mechanism to stop them from loosening or tightening during riding. However the nylon’s ability to grip the thread slowly degrades, especially when it is taken on and off the thread. This degradation can cause the nylon to no longer do its job, potentially leading to parts falling off. Check the condition of axle nuts during bearing maintenance, as winding them off to check them will wear them out quicker. When the nylon is degraded the only answer is new nuts, fortunately they’re cheap and readily available. The kingpin is 3/8-24tpi and the axle is 5/16-24tpi.
2. Pivot cups - The hole in the pivot will eventually enlarge, this usually takes quite a long time, but it can cause stability problems when it gets bad.
3. Bushings - The rebound of the bushings will degrade with use, this is unavoidable as the molecular chains slowly break when they get stretched and compressed. While low rebound isn’t necessarily a serious problem, the truck will feel and perform better if the bushings are replaced when they lose rebound.
4. Kingpins - These endure a lot of force and vibration and literally hold the whole truck together. Unfortunately it’s practically impossible to assess their condition. It is therefore advisable to replace them every year.
All prices are in USD.