I got two new impellers today for my old ted willams and had a question. Do I have to have the impeller in place on the bilge pump side of the water pump housing? Or do they both have to be in place? If they have to both be in place can I just remove the pin so the upper one doesn't spin? I would like to just run the impeller for the motor cooling and save the second one in case the bottom one goes out. I figure it's easier to buy a bilge pump if I need it than find a another impeller for this motor. Thanks