How do you rebuild a lower unit on a 40hp mercury outboard?

To rebuild a lower unit on a 40hp Mercury outboard, start by removing the lower unit from the engine by unscrewing the bolts and disconnecting the shift linkage. Drain the gear oil and disassemble the lower unit, noting the arrangement of parts. Replace worn components such as seals, bearings, and the water pump, then reassemble the unit, ensuring all parts are properly aligned. Finally, refill with the appropriate gear oil and reinstall the lower unit on the outboard.