When I had to replace my water pump assembly the impeller had radial play, in other words the impeller needed to be bolted down tighter. The aluminum impeller mounting hole has a flat spot that had developed/worn allowing the extra play clockwise an visa versa. This is only my opinion and not to diss anyone, I believe that doing Jerry recommendation would help. But I did not perform that mod (I didn't hear of the mod until after I replaced everything) and my pump worked as designed. When the fisherman was doing the dyno runs his water pump was leaking, after tightening down the impeller bolt it stopped and we were able to complete the runs. I am not an expert on this so please do not take my post as an insult to anyone