Replacing outboard gearcase lubricant

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It is important to change the gearcase fluid on an outboard motor at least once a year or every 100 hours of use, whichever comes first. It’s time to get out the drain pan, buy some new gearcase lube and refill the lower case unit. Lubricants break down over time and the lubricating properties diminish, causing added friction within the gearcase, which in turn causes heat and increased wear on the moving parts. Over time there is also a buildup of metal shavings and other pollutants that decrease the lubricating efficiency of the gearcase oils. Join us as we get up close and personal and change the gearcase lube on the lower unit.

What you’ll need:
• A large flat screwdriver to remove the drain and vent plug screws.
• New O-rings and sealing washers for the vent plug and lower unit drain screws.
• Manufacturer recommended lower unit lube for your particular motor.
• A lower unit pump and lube tube.
• A catch pan for the used oil which will be removed from the lower unit.
• A container to transport the used lower unit oil to a recycling depot.
• Clean rags for wiping excess oil and any spills that occur.
• Disposable gloves.
• New gearcase lube.

Replace the drain screw gaskets every time you replace the gearcase lube to ensure the unit is sealed properly and does not allow in any water. Water, particularly sea water will stick to the gear, shafts and bearings and begin to corrode them. It does not take long for this corrosion to cause the unit to fail. In colder climates, water contamination which freezes can crack the gearcase housing as it expands.

outboard gearcase


1. Use the appropriate screwdriver, which is large enough, so as not to damage the screw slot. The lower drain plug screw is usually most often situated on the starboard side of the lower unit, beneath the bullet-shaped portion of the lower housing. Locate your drain plug/fill screw and adjust the trim of your outboard so that this plug is in the lowest position possible. This allows for the used lube to be drained thoroughly. Often, due to the differing metals in the drain plug screw and the housing, it can be difficult to remove due to corrosion. Using a penetrating release agent will help to remove the drain plug screw.

2. Fill the gearcase with the new lube slowly and keep an eye on the lube coming out of the vent hole. If there are no air bubbles present, it indicates that the gearcase is full.

3. Replace any drain screw gaskets with new ones each time you do a lube change on the lower unit.

4. Once the unit is full, indicated by lube exiting the vent hole, go ahead and replace the upper vent screw. Now replace the lower drain screw as quickly as possible so as not to lose too much lubricant, and then wipe the area clean.
Gearcase fluid that has been contaminated by water will have a milky complexion. Burnt gearcase lube will have a blackish colour to it and will have a burnt smell. This usually means the gearcase is not set up correctly or there is excessive wear and the consequent buildup of heat in the gearcase causing the lube to burn. Many gearcase drain screws are equipped with a magnet to collect any metal shavings that might occur within the unit. It is common for there to be a slight buildup of metal shavings/filings on the magnet, but not a significant amount. If there are a large amount of filings on the magnet, this might indicate an improper wear problem in the gearcase.
Whether you are doing a simple yearly lube replacement or you are replacing burnt lube you need to completely remove all the old lube so as not to contaminate the new lube in any way.

All manufacturers indicate in the owner’s manual which lower unit lube should be used. If you’ve bought used outboard motors, often a simple search online will indicate which lube is recommended by the manufacturer. If you don’t have any success online, a call to a local servicing agent in your area will usually uncover the information you need.

Once the lower unit lube is replaced, pull off the prop to check for any tangled fishing line and remove. Grease the propeller shaft well and then replace the propeller. Touch up any nicks and scratches on the lower unit to prevent corrosion, and then take you boat out on the water for a leisurely cruise.


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