Part # 3 is the CAM and it is FIXED inside the pump. It is constantly in contact with the blades of the impeller, bending them so there is less volume of water which is providing the “pump action”. If it wears down it is only because of contact with the rubber impeller and not with the pump case. (Look at the photo and see the compressed area under the cam)

I had the experience of the BACKING PLATE (part # 8) that completely disappeared after 12 years of service and badly affected the pump performance. That is a rather thin part and I can understand how it will be affected by the constant contact with the blades.

As to the volume per minute (flow) All I contended is that you do not know the resistance of the overall engine passages so a “free flow” of the pump will not tell you much if you do not allow the “resistance” of the engine to be in place. On the other hand if the pump is connected to the engine then all you cam measure is at the end after the exhaust and that is affected by all the “circuit resistance”. If I was a betting man I would put the injection elbow into the exhaust very high on the list of potential “resistance” points that should be cleaned. My experience was that after 12 years of use that area corrodes and builds dirt. My solution was a new SS fabricated exhaust that solved that problem. If your engine is 20 YO then that area should be looked at.

Good luck!