Lower Unit

Keep the Gears in Motion

The lower unit on a boat converts the power of your outboard’s engine to rotation of the prop shaft, which moves the propeller, which moves your boat. Without it, you go nowhere. Its internal mechanical workings put tremendous pressure on the lubricant inside.

This hardworking device just needs some simple, regular maintenance to help it stay that way.

How to Maintain the Lower Unit

Check for Damage
Visually inspect the exterior of your lower unit for any signs of damage every time before you go out. If you fish, it’s a good idea, every 50 hours or so (or when necessary), to remove the propeller and check for any fishing line or other debris that may be wrapped around the propeller shaft. This can cut into prop shaft seals and potentially allow water to enter the gearcase.

If found, here’s what you should do:

Visually inspect the prop shaft seals for damage or leakage.
Check the color of your lower unit lubricant by cracking the “drain screw” on the bottom of the bullet and the “vent screw” on the side of the lower unit.
Allow a small amount to drain into a clean container. If the lubricant is milky in color, you’ve got water in the lubricant.

Have the lower unit seals inspected by your Yamaha Marine dealer through a pressure test, and if necessary, have the seals replaced. Pressure testing a Yamaha lower unit is best performed by an authorized Yamaha Marine dealer, because depending on findings, it may be necessary to disassemble the lower unit for further inspection.

Tip: It’s best not to run the outboard right before checking the lower unit lubricant, as this will agitate the oil with whatever air is trapped inside. The resulting fine bubbles might be confused with the milky appearance that occurs with water ingestion.

Change the Lubricant
Yamaha recommends replacing your lower unit lubricant after the first 20 hours for a new outboard, then every 100 hours after that. Also, if it’s milky in color or smells or looks burnt when you’re checking it, it’s time for a change.

Here’s how to change the lower unit lubricant:

Place a pan under the bottom drain screw, remove the screw, and then remove the upper vent screw on the side of the lower unit. Allow all of the lubricant to drain into the pan.

While you’re waiting, check the drain screw. It’s magnetic, so any metal particles or shavings present in the lubricant will be attracted to it. Fine metallic dust is normal, but if you find larger chips of metal on the screw or in the pan as the lubricant drains, have your local Yamaha Marine dealer check for possible internal damage.

Also at this time, replace the small crush washer present on each screw (commonly known as “drain plug gaskets”). If you don’t see one, it may still be adhering to the outboard. Use a pick to pull it free, as these must be located, discarded and replaced each time these screws are removed.

Tip: These drain screw crush washers serve as gaskets and are not reusable. They are a main source of protection against water intrusion into the lower unit, which can cause catastrophic dilution of the lubricant. Do NOT try to save a few pennies here. Lower units are very expensive. It’s best to always have some extra drain plug gaskets on hand.

Once all the lube has drained, refill the lower unit by threading the appropriate fitting into the lower unit drain screw opening (on the bottom of the bullet) and slowly pumping in fresh Yamalube Marine Gearcase Lube or Lubricant HD.

Once you see this new fluid seeping out of the upper vent screw hole, stop and wait 5 minutes, then very slowly continue pumping until it seeps from the vent screw again. At this point, replace the vent screw with a new crush washer installed, and tighten according to manufacturer specifications.

Remove the fitting attached to the drain screw opening, and quickly re-install the drain screw with the new crush washer installed. Tighten to factory specifications.

You may choose to do these procedures yourself, or you can have your authorized Yamaha Marine dealer do them for you. Either way, performing them is important for proper continued operation and your continued satisfaction and enjoyment.

Videos:

Lower Unit with Steve Pennaz

Your Outboard at Your Fingertips

The MyYamahaOutboards App
Dealer-completed service. DIY maintenance. Track it all with MyYamahaOutboards. It puts your complete outboard maintenance history at your fingertips. Plus, you can keep custom trip logs and checklists, request service and so much more.

Learn More About Our Free App