How often should I change my gear case oil?

When Should I change my gear oil in the gear case or leg of my boat?


The gear case oil is often overlooked in today’s engines, most manufacturers recommend at least once per year.  Some differ with the new Evinrude E Tec only needing the gear lube change every 3 years (on some models)

Gear oil is the life blood of your engine’s drive train, it provides both the transmission and final drive function. Gears and bearings need this lubricant to last and perform the job they were meant to do.

We recommend changing your gear oil every fall before freeze up, if water has gotten into the chamber it could freeze and cause expensive damage to the internals, often resulting in a cracked housing. This kind of repair can cost thousands of dollars and most are not covered by insurance. If water is left to sit in the housing then it will allow the gears and bearings to start to rust, this may not be apparent right away but over time can cause a bearing to fail and take out the gears with them.

Some drain the oil out and leave it empty over the winter, this is also a problem as condensation will form and allow rusting of parts as well. Drain your gear oil and check the lube coming out, if it is “milky” looking or white, it has water mixed into it, this needs to be investigated. Many times when we see this condition it’s caused by loose drain & fill screws or the lack of new drain plug washers used when changing or the lack of any at all. Take your prop off and remove the thrust washer to see if fishing line has got caught in the seal, this will also cause leakage or water intrusion.

If you find the gear oil is water contaminated you need to identify how it got into the case. The easiest way is to do a pressure & vacuum check to see what is leaking. This is easily done by the shop and can prevent further damage. When the leak is identified then you can either change the problem seal or have all the seals replaced, this is a good idea because it allows for inspection of the components to see if any damage has been done.

Another common problem when draining gear lube is when you have chunks of metal coming out when the oil is draining, this is a serious problem usually resulting from impacts to the propeller or component failure.

If your gear oil is black, then it’s just time for a change, be sure to replace the drain plug washers and tighten the drain screws to proper torque. Filling a gear case can be tricky but with the proper tools it’s not that hard. There are pumps that fit onto bottles that allow you to pump the oil back in. Gear cases must be filled from the lowest point (often the drain screw) to allow the air to be purged and allow a proper fill. Adding oil from the bottom until a steady stream of oil comes out the vent screw (often marked “level”) will get this done. Once you have all the air out, replace the top screw and tighten, this will cause a vacuum in the gear case and you then can unscrew the pump out of the bottom hole and place the screw in and tighten. Once you have the bottom screw in you can check the level by removing the top screw again. Follow you manual for gear lube type and quantity.

If in doubt we can do this for you, each engine is slightly different and the procedure, although it is common some have slight variations.