Have a 14 month old RIDGID SP500 sump pump, and the #10 tie rod screws that hold top assembly to the cast iron volute have rusted out and parted at the volute housing. Made new bronze pieces, but before dissasembly, need to know how the cast iron impeller is fastened to the motor shaft. Threaded or set screw ?? Catalog exploded view does not have enough detail. Anyone done this at home?