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Lower Unit Outboard Motor Maintenance

lower unit
Your lower unit is the difference between cutting through the waves and being dead in the water. Take good care of it with Blain’s Farm & Fleet.

The lower unit (or “foot”) of your outboard motor is the bottom part that houses the prop and gear case. Taking care of your lower unit is vital to making sure your outboard runs smooth and has a long life. The main keys to lower unit maintenance are keeping fishing line and other debris from getting wound in the prop, and changing your lower unit oil regularly.

How to Change Your Lower Unit Oil

Always consult your owner’s manual before changing your own lower unit oil. This will tell you the type and weight of gear lube that you’ll need, and may even include a diagram and instructions on how to change it. If you run your motor for fewer than 100 hours during the season, change your lower unit oil once per year, right before you store your boat for winter. If you run your outboard for more than 100 hours, change your lower unit oil every 100 hours.

What you’ll need: 

Step 1. Put your motor in the vertical position.

This is the same position it would be in in the water, with the prop shaft parallel to the ground. Lay the cardboard on the ground under the motor.

Step 2: Put the oil pan under the prop and remove the lower drain plug. 

Use the screwdriver to unscrew the drain plug. Some marine mechanics recommend using an impact driver to reduce the risk of stripping out the threads if the plug is damaged or over-tightened. Lower unit plugs are magnetic so they can gather metal shavings to give you an idea of how much wear is happening. Check the plug for metal shavings. If it has a few here and there, it’s no problem since some wear is normal. If it’s almost completely covered, take the motor to a marine mechanic as soon as possible.

Step 3: Remove the top plug. 

This will vent the lower unit and allow the oil to drain faster. Some outboards have two top plugs. If yours does, make sure you remove both of them while you change your lower unit oil. Never remove the top plug before the bottom drain plug, or you’ll make a mess.

Step 4: Inspect the drained oil and your oil plugs. 

This step is key, but often gets overlooked. The oil should be dark and not milky. If yours is milky, it means you have water in your oil, and you’ll need to take your motor to a marine mechanic. This is usually caused by a leaky seal due to fishing line or other fine debris that got wrapped around your prop shaft. If you have water in your oil, don’t finish the oil change because water will just seep back in when you get out on the water. This will do more damage to your gear box.

Your oil plugs have gaskets. If these gaskets are frayed or they look worn, replace them. Also, if the gaskets come off the plug and stick in the plug hole, you may want to replace the plug as well. You don’t want to risk getting water in your lower unit.

Step 5: Once the draining oil reduces to a drip, refill your lower unit through the bottom drain hole. 

Thread the tip on your lower unit pump into the lower drain plug hole. These pumps come with adapters to fit metric or SAE threads. Leave the upper plug(s) out while you fill. Pump gently and slowly to avoid getting too much air in the gear case. Once you see oil start to trickle out of your top vent hole(s), your lower unit is full and you can stop pumping.

Step 6: Quickly remove the pump nozzle and replace it with the bottom drain plug. 

Snug the plug up well (your owner’s manual may include a precise torque specification for the drain plug. Wipe it off afterwards and make sure no oil is leaking from the plug.

Step 7: Replace the top vent plug(s). 

Thread them into place and wipe up any oil on the outside. Tilt your motor up and make sure no oil leaks out of your upper plug(s).

Step 8: Clean up any spilled oil that the cardboard didn’t catch, and dispose of the waste oil, used oil absorbent, and cardboard properly. 

Take them to a waste disposal facility to prevent pollution. If you don’t know where the proper facility for oil disposal is in your area, call your local municipality and they should be able to help.

Inspecting Your Lower Unit

You only really need to do a visual inspection of your prop and make sure it’s clear of debris. If you find water in your oil when you change it, you can pull the prop out and inspect its shaft and seals. This, however, is a fairly involved job that usually requires professional know-how.


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