I am replacing the impeller on this folding shaft motor and I have seen on various web sites mention of an O ring on the shaft. When I took off the lower unit there was not an O ring. There is a round rubber seal just inside the lower unit that is surrounded by a round brass piece that is pressed into place and the shaft runs through this rubber seal. Do I need to replace this seal/ring and if so does anyone have a part # to reference? Also, are there any helpful hints for installing the impeller? Thanks