The biggest challenge was how to keep the roll pin in place as I slipped the impeller onto the shaft. A dab of silicon did the trick. After it setup it kept the pin in place and I fit the impeller back on. I reassembled the layers of the pump in reverse order and replaced the insulators with some homemade insulators made from electrical tape.
Next up was replacing the lower oil. It is recommended to do this at the end of each season. My oil was quite dirty so this was a well deserved service. I drained most of the oil before pulling the water pump but more came out when I removed the drail hole and vent hole screws. The McCulloch uses the standard Mercury fitting that came on the pump from West Marine. Also the Mercury Lower Unit plug gaskets fit perfectly.
The lower requires 15 oz of gear lube. After it started "weeping" from the vent hole I kept pumping (about another 10oz of fluid) until the gear oil started running clean.The old oil was black and dirty!
With the new impellers I've got great circulation and a strong tell-tale stream. (You can't really see the tell-tale in this photo but it is there.)