Why do Motorcycle Stators Fail?
Motorcycle stator failures have many causes, though they always have one thing in common: the coil windings. The coils of the stator can either fail open, meaning the wire is physically broken and disconnected, or closed, where the wire shorts through it's insulation to a neighboring loop of wire, or to the metal core of the stator, mounting bolt, or engine. Either type of failure will result in a bad stator that needs to be replaced. However while an open failure will completely eliminate any output from the damaged coil, a closed failure can result in the stator still operating, though with significantly reduced output and performance.
Motorcycle Stator Heat Buildup
A stator's biggest enemy is heat buildup in the wire windings. While all stators use wire with high temperature handling insulation, the quality and exact temperature ratings are very important. It's critical for reliability to use stators built with insulated cores, high quality 200°C rated wire insulation and high quality coil winding processes to ensure even tension of the coils. Heat can build up in, and damage the windings from many sources. A motorcycle engine that runs hot should be fixed as soon as possible. High engine temperatures can lead to stator coil open and closed failures, especially in modern motorcycle engines that submerge the stator in an engine oil bath to help dissipate heat. A failure of the rectifier function in the voltage regulator (also a part of the motorcycle charging system) can also cause high heat buildup in the windings, by preventing the stator's power from being consumed by the motorcycle's electrical system.
Motorcycle Stator Physical Damage
The other common stator failure is from physical damage to the wire windings, either by damaging the insulation or the copper wire itself. A stator replacement can certainly be accomplished at home with basic tools, but an inexperienced mechanic, or sloppy installation procedures can lead to damaging the stator's wire or insulation. Even a small scratch in the insulation can be enough to quickly cause a short in the windings as the wire heats up from use. It is also extremely important to ALWAYS use threadlocker on the stator mounting bolts, since the vibrations from the motorcycle's engine can cause them to back out of their threads, quickly destroying the stator and flywheel.