If you look closely at the picture above, you can tell that the motor flange is not sitting evenly in the seal plate of this whisperflo pump. The owner called me out because his swimming pool circulation pump was not working. I found the typical symptoms; filter pump breaker tripped, motor just hummed when circuit breaker was reset, impellor frozen. One might suspect the motor had gone bad, unless they had experienced this before. And I have. More times than I can count. Somehow, the combination of brass sealplate inserts, stainless steel bolts and the aluminum endbell of the motor cause the aluminum to corrode and expand, pushing the motor away from the seal plate. Oddly, it never affects all four motor mounting bolts evenly, usually only one or two, which causes the pump impellor to go cock-eyed and bind against the seal plate. You can see this in the picture. The unfortu ate part is that although the motor is still good, the pump needs to be dissassembled the same as if the motor were being replaced. And usually, the bolts are seazed so badly that the seal plate ends up getting destroyed in the dissassembly process. Sometimes, the motor end bell is so corroded that the motor has to be replaced anyway. I know how to fix this, but the question that remains is why does this happen? I have only experienced this on whisperflo pumps, but I have been  seeing it for more than 10 years.