How to Change the Lower Unit Oil on a Boat

Whether you have an outboard or a stern drive, changing the lower unit oil is a regular maintenance chore every boater can do.

19th July 2012.
By Lenny Rudow

Few maintenance chores are as important as changing the lower unit oil on your boat’s outdrive, on a regular basis. Sure, you could pay a marine mechanic to take care of this, but it’s a simple process which you can do right in your driveway or in the boatyard, saving both time and money. You’ve never tried it before? No worries – we’ll walk you through the process, step by step, here and in our oil changing video.

Step 1. Gather all of the tools and materials you’ll need, before you get started. This will include:

  • Large, flat-head screwdriver
  • New lower unit oil, of the engine manufacturer’s recommended brand and type
  • Drain pan
  • Rags to clean up spills
  • Lower unit oil pump and any necessary adaptors/fittings

Step 2. Locate the lower and upper drain holes, which will be plugged with a screw. After placing the drain pan below the lower unit, remove the lower screw. Oil will begin slowly draining out.

Step 3. Next, remove the upper screw. This will allow air to flow into the lower unit, displacing the oil, and the remainder will drain out quickly. Tilt the drive unit up a bit if necessary, to be sure all the oil drains out.

Step 4. Visually inspect the oil. If it’s a milky color or you can see water in it, that means there’s a leak somewhere in the system—stop right there, and take the boat to a marine mechanic for service before you use it again.

Step 5. Assuming the oil looks good, screw the oil pump’s fitting into the lower drain hole.

Step 6. Attach the pump to the jar of fresh oil, and start pumping. If your lower unit takes more than a quart, you’ll have to swap it out mid-stream. Continue pumping in oil until it begins flowing out of the upper drain hole.

Step 7. Plug the upper drain hole first, with the drain plug screw. Then remove the pump fitting from the lower drain hole. Oil will dribble out slowly since the upper screw restricts the air flow, but you still want to get the lower screw in as quickly as possible to minimize how much comes out. (You may want to use a finger to plug the hole, as you remove the oil pump fitting and reach for the drain plug screw and screwdriver.

Step 8. Wipe off the lower unit, and get all of that oil soaked up. Remember: anything you miss will end up in the water.

Step 9. Bottle up the used oil, and take it to the appropriate recycling facility.

Step 10. Re-launch your boat, fire up the engine, and head off to new adventures and distant horizons—it’s time to enjoy your well-deserved day on the water!

Get the engine parts you need for the job at our Gear and Parts Store.

-Lenny Rudow


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About the author:

Lenny Rudow

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Lenny Rudow is Senior Editor for Dominion Marine Media, including Boats.com and Yachtworld.com. With over two decades of experience in marine journalism, he has contributed to publications including Boating Magazine, Marlin Magazine, Boating World, Saltwater Sportsman, Texas Fish & Game, and many others. Lenny is a graduate of the Westlawn School of Yacht Design who has won 28 BWI and OWAA writing awards.
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