Why the starter continues to go out?

There probably are many causes, but the one which I have observed several times, and therefore seems most common, is abnormal tortional stress caused by misalignment of the starter to the flywheel. It is very important that the axis [centerline] of the starter shaft be precisely perpendicular to the face of the flywheel. If the starter is misaligned, the uneven loading of the teeth of the starter gear,and the gear teeth on the flywheel will cause abnormal wear on the flywheel and starter drive gear teeth, and excessive excentric loading on the starter bearings and commutater. The TBF [time before failure] depends on the seriousness of the abnormal loading and wear, and in severe cases, failure of the starter can occur within hours of use.