How to Change the Lower Unit Marine Gear Oil

The lower unit on a marine motor describes the bottom section of the motor that contains the transmission gears and propeller shaft, which typically sit at or below the waterline. For any marine motor to work properly it must contain the manufacturer's prescribed brand and weight, or viscosity, of gear oil designed for the specific motor. The lower end gears and shaft endure a lot of heat and friction, especially when the motor operates most of the time at full throttle. The proper way to change the lower unit gear oil requires some basic steps and a few tools.

Instructions

Remove the boat from the water, if you have docked. Trailer and transport the boat to a suitable work location that has a level surface. As a precaution, use a socket and wrench to remove the negative battery cable, if so equipped. Remove the ignition key from the dash. Tilt the motor in a straight up and down vertical orientation -- the motor should not be trimmed up or down in any fashion.

Refer to your owner's service manual for the location of your bottom gear drain plug and the upper vent plug on the lower unit. Both locations might be marked with white or red paint and contain slot head screws recessed into the motor case. Your motor might have Allen screw heads instead of slot head screws. Place a drain pan directly under the lower unit.

Use a slot screwdriver to loosen and remove the bottom drain plug first. Let the oil drain into the pan for several minutes, then loosen and remove the top vent drain plug. Removing the vent plug will release the vacuum and allow the oil to drain faster. Let the oil drain for 10 to 15 minutes. Be wary of any oil that looks cream-colored or frothy -- this will indicate a problem with water contamination in the gear case.

Pour your replacement oil into a hand pump bottle that has a special adapter to fit over the oil drain plug opening. If you have a large pump bottle, fill it to capacity with new oil as outlined by your service manual specifications. Place the hand pump hose adapter over the drain plug opening and begin pumping oil into the lower unit. Pump until you see the top vent plug overflowing.

Pump three more times after you see the overflow from the vent hole. Quickly screw in the vent plug with one hand while holding the adapter hose against the drain plug. Tighten the vent drain plug with the screwdriver, or Allen wrench. Quickly pull the adapter hose off the drain plug and insert the drain plug, screwing it in by hand. Tighten the drain plug with a screwdriver or Allen wrench. Wipe down the side casing with rags and remove the drain pan. Reconnect the negative battery cable and tighten it with a socket.