The generators in Hoover Dam are made to repaired as needed. The water wheels in all of them are in the process of being replaced. The bearings run in full hydrodynamic lubrication which means there is no metal to metal contact. The generator slip rings wear because of the brushes riding on them, but they are designed to be replaced as needed. The generator winding insulation degrades with age and electrical stress, but can be replaced when it fails. etc. They are big old machines that are well made but don't qualify as high precision in the sense of a watch or even a shotgun.

OB