Today, I pulled the bottom unit to replace water pump on my 75 hp force mercury (1996) motor. Lowered motor to vertical, went into shop to tear apart water pump, came back out and found a part laying on my tarp. This part does not match a single part from my diagrams. it is 1 in in dia. and 1/2 inch thick made from hard plastic, that has groves cut into it like it belongs on a shaft. After I finished repairing water pump, it would fit on top of shaft and slides down to end of keyways but it was not on there when I took it off. Any help would be great, don't like having spare parts left over.