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TRAVEL BOATING How to Replace Outboard Gearbox Oil

How to Replace Outboard Gearbox Oil

By: Geoff Morrish | Aug 23 2010 | 635 words | 920 hits

How do you change the outboard gearbox oil? Or lower unit oil?

It is very simple and easy to change the gear oil or gear lube in the lower unit or gearbox of your outboard motor. The most important thing to remember is to fill the gearbox or lower unit from the bottom drain hole.

A lot of people struggle at first to accept this but it is most important. If you fill the gearbox from the top hole, unless you are VERY LUCKY you will get an air lock in the gearbox, this will be a big bubble of air that takes up the space required for the gear oil to fill. The oil will start over flowing out of the top hole and you assume that the gearbox if full and stop filling it. A gearbox without the correct amount of gear lube in it will fail very quickly.

How To Replace Outboard Gear Oil

  • With the outboard motor still tilted up, loosen both drain screws. It is easier to loosen the bottom screw with the outboard tilted up.
  • Tilt the outboard down as far as you can. If the prop shaft can't be horizontal, or angling down, you will need to drop the front of the trailer or jack the back of the trailer up so you can lower the motor.
  • Remove outboard drain screws and drain oil into a container. There will be two screws, one at the bottom of the gearbox on the side and one at the top of the gearbox, usually on the same side as the bottom screw.
  • Inspect old gear oil for signs of metal or water contamination. Most outboard motors will have small magnets on the lower unit drain screws. Inspect these magnets for signs of metal. If it doesn't have magnets, look at the oil in the sun, metal filings will be shiny in the sun. Water will either drain out first before the oil, or it will have mixed with the oil and made the oil go a white milky colour. If you have water in your gearbox you will need to have the gearbox pressure and vacuum tested to find out which seal is leaking and have it replaced.
  • Before filling the gearbox, make sure both of the old drain screw washers have been removed and all of the pieces are cleaned from around the thread, on both of the drains screws, and the holes in the gear box.
  • Fit a new seal or washer to each drain screw.
  • Fill the gearbox with outboard motor gear oil only.
  • Fill the gearbox from the bottom hole, you can buy a bottle of gear oil and a pump relatively cheaply from marine dealerships.
  • Pump the oil until it comes out of the top hole, wait a couple of minutes to let any air bubbles work their way out then pump until the oil comes out of the top hole again.
  • Fit the drain plug screw into the top hole first and tighten it securely.
  • Remove the pump from the bottom hole and fit the drain screw and tighten securely. If you need to tilt the outboard motor up so you can get a good grip on the bottom screw. Make sure they are tightened securely.
  • Wipe any excess oil and you are all done!

One important note is to make sure you purchase the correct seal or washer for your make of outboard. Although some seals from different manufacturers look as though they go on correctly, they can be slightly larger in outside diameter. When you tighten them in your gearbox the washer will tighten around the outside of the thread and not actually bottom out. This will allow gear oil to leak out, or let the water get forced into the gearbox under the high pressure when the boat is moving through the water.

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    Replace Outboard Gearbox