Replacing Lower Unit Oil


At a minimum, lower unit oil needs to be replaced once a year. As always, common sense needs to be applied to this. My common sense tells me that if I replace my lower unit every fall, when my boating season is over, I accomplish 2 things. First, my oil gets changed, which is important. Second, if there’s any water in the case due to bad seals, I don’t leave the water in over the course of the winter. If you’re in a state where things freeze over the winter, water in your lower unit will freeze, expand, and may crack the gearcase. That’s expensive. 10 dollars for lower unit oil or hundreds and hundreds of dollars for a replacement lower unit? Common sense, right? As an add-on to the 2 things a fall replacement of lower unit oil will provide, you also get an opportunity to see if there is water in there and if your gearcase does need to be resealed, you can deal with it during the months when it’s not cutting into your boating time. If that’s still not enough, many shops run a summer rate and a winter rate and that repair is cheaper in the winter than it is in the summer. Take my word for it-replace lower unit oil every fall, after your last fishing trip. You’ll be glad you did.

I’m going to save you 30 or 40 dollars here. Since you’re changing your lower unit oil in the fall, I’d encourage you to spend the money on a Christmas present for your significant other. Remember, she’ll love a good set of bass jigs at the holidays.

First, you don’t need to drop your lower unit in order to change the oil. The plugs on lower units are on the outside of the unit so if all you are doing is the oil, you can do it while the lower unit is still installed. With that said, remember that you can use this opportunity to change the impeller, which will be discussed in a different instructional. An impeller inspection or change would require you to drop the lower unit.

What you need:

  • A flathead screwdriver, in most cases. Older Force outboards used a allen wrench (possibly the stupidest design ever created, which you’ll find out when they strip out). Some Suzuki’s used a # 2 Phillips head screwdriver. To my knowledge, everything else uses a large flathead screwdriver.
  • Oil. There are different recommendations from different manufacturers on the brand, of course, but the bottom line is that you need 90 weight gear lube. You can buy this at any marine business, various websites online, Wal-mart, and even some auto parts stores. We have a local hardware store with a boating section that carries it. I’m not recommending a brand, but the bottle will say that it’s outboard gear lube and it will say 90W on it.
  • Seals, for the drain plug. They’re just fiber o-rings, to keep it simple. Each manufacturer uses different ones, so please check your service manual for the correct ones for your engine. You’ll need 2, in most cases.
  • A pan, to drain the oil into. ***Recommended*** A lower unit fill tube. This screws into the top of the one quart lower unit oil bottles and allows you to pump oil into your lower unit. I know they sell squeeze tubes of oil. What a pain in the butt. Splurge for the extra 9 or 10 bucks one time and you’ll have the fill tube forever.

What you do:

  1. Position your pan under the lower unit.
  2. Remove the bottom oil plug. At this point, there’s suction on the oil and not much will come out.
  3. Remove the vent plug, which is higher up on the side of the unit. This will release the suction and the lower unit can drain.

    ***At this point, observe the oil that’s draining out. Keeping in mind that oil and water separate, inspect the first oil that comes out. If it’s milky, you may have a little water in there. Often as not, water comes out first. Since this is a sealed unit, if you have water coming out first, you have a seal leaking and may want to see a mechanic.

    The lower unit will take a couple of minutes to drain. Grab a cup of coffee, or something to that effect.

  4. Place your new seals on your drain and vent plugs at this point. Remember to remove the old seal first, of course. Set those aside, close at hand.
  5. If you took my advice and got a fill tube, screw that on to the quart of oil. Thread the other end into the bottom hole (drain plug hole). If you did not, then insert your squeeze tube into that hole. You will want to fill the case with oil until it comes out the vent hole.
  6. When you’ve seen a little oil come out of the vent hole, stop filling. Don’t remove the fill tube or squeeze tube yet. Keeping it in the bottom hole, go ahead and thread the vent plug back in (top hole). This, again, creates a suction that will keep your oil from coming out the drain hole when the tube is removed.
  7. Assuming that your vent plug is back in place, you can remove the tube from the drain hole and quickly reinstall the drain plug back in place.
  8. The vent plug and the drain plug should be hand-torqued with the screwdriver. You don’t have to wrench it down. Just past hand tight will do the trick.

Congratulation! You’re done, and your lower unit oil is ready for the next boating trip. Properly dispose of old oil, wash your hands, and start figuring up ways to spend the 30 or 40 dollars you just saved by putting in 10 minutes of your own work.