Originally Posted by capecodda
Since then, I've put a zinc on the shaft, forward of the donut to serve as an emergency stopper for the donut. Belt and suspenders.
There are two set screws on the rotor that are intended to "bite" into the shaft to keep it in place. If they loosen, the bellows can push the rotor back on the shaft, thereby allowing water to leak through and be spun off as the shaft rotates--hence the lawn sprinkler comment.
In theory, the second set screw would back up the first set screw, but I like the idea of another backup. The shaft zinc would work if it can really be clamped down on the shaft. It might be a good idea to sand down the mating faces of the zinc to make sure you can really clamp down with enough force to assure it would stay in place.
Considering the actually force produced by the compressed bellows, it probably doesn't take much to back up the rotor, so that the unmodified zinc of the proper size is probably quite adequate. But if you really want peace of mind, make sure the zinc is clamped tight.