Once every 100 hours or once every year, Mercury says you must remove your lower unit to inspect the condition of the lower unit and water pump, and change the oil. The inspections aside, how to remove and replace the lower unit on your Mercury outboard motor is a process as simple and straightforward as taking off a boot and putting it back on.
Move the shifter into "Forward" gear. Remove the lower unit locknut and washer from the anticavitation plate, using a 12mm open-end wrench.
Remove the bolts, two on each side of the lower unit, and washers between the lower unit and the exhaust housing, with a 14mm open-end wrench.
Pull down on the lower unit to separate it from the housing. Guide it straight out of the exhaust housing along with the shift rod and driveshaft. Place the lower unit in a bench vice.
Remove the old gasket material from the fill and vent recesses and from the vent and fill screws with the corner of a putty knife blade.
Apply a thin film of waterproof marine grease to the inner diameter of the water tube seal, to the shift shaft coupler and to the driveshaft splines. Don't allow the grease or oil to get on the ends of the shafts or they may not fully seat themselves in the shift coupler and crankshaft.
Apply thread locker to the threads of the four 14mm bolts that hold the lower unit to the exhaust tunnel, but do not get thread locker on the locknut that will be installed under the anticavitation plate.
Align the gear shaft, shift shaft and water tube with their counterparts in the exhaust tunnel and lift the lower unit straight up. Thread the lower unit bolts into their bores as tight as possible by hand. Tighten them evenly to 40 foot-pounds with a torque wrench. Thread the locknut into place and tighten it to 22 foot-pounds.
The first time you operate the motor after reinstalling the lower unit, check the lower unit carefully for leaks; Loosen the drain screw with a screwdriver and the vent screw and drain a teaspoon of lubricant. Check the lubricant for a whitish appearance. Dab a finger into the lubricant and rub the lubricant between your finger and a thumb, feeling for metal particles.
When you're working on your motor, remove the nut from the negative battery post, marked with the letters "NEG" or a minus sign, and lift the cable from the battery. This isolates the electrical supply from the boat's common ground, allowing you to work on the boat's electrical or mechanical systems without fear of the motor starting or another system being energised. If you work on your outboard motor when your boat is on its trailer, or your motor is on a storage stand, remove the propeller nut with a 1 1/16-inch wrench and slide the thrust hub, propeller and washers from the propeller shaft. Failure to remove a propeller before operating an outboard out of the water during maintenance or long-term storage is an invitation to a propeller-strike injury, which can maim or kill.