Change the Gear Oil in a Used Stern-Drive

The gear-to-gear mesh in a stern drive or in an outboard lower unit is as hard on a gear oil as you would expect. Shearing breaks down the oil, which means it should be one of the first things you change on a used boat you just bought.

The gear-to-gear mesh in a stern drive or in an outboard lower unit is as hard on a gear oil as you would expect. Shearing breaks down the oil, which means it should be one of the first things you change on a used boat you just bought.

One of the first things I do when I buy a used anything with an internal combustion engine is change the fluids. Engine oil goes first. Then the transmission fluid, if it has a transmission, then the differential on a car and the gearcase on a boat. I use synthetic oil, of course.

Changing the oil in the drive gives me an opportunity to inspect the magnets for metal debris and the oil for possible water contamination. Finding problems early before they become major could offset a possible future catastrophic failure. It’s also important because there’s so much “oil shearing” that takes place inside a gear case. Shearing results from the gear-to-gear mesh, and once an oil has sheared, it cannot heal itself. The oil won’t look any different, but its chemical makeup has been altered, even if you use synthetic.

“On an atomic level, the polymers are going to look like a bunch of little rubber balls and they’re just suspended throughout the base oil with the additive package, and that’s what’s going to give you your resistance to flow at a given temperature, which is viscosity,” said Jared Martin, regional sales manager for Royal Purple Lubricants. “And going through the gears, they’re going to get sheared up. Now you’ve got a bunch of molecules that are imperfect in size, or you get several different sizes, and it’s going to alter the viscosity of the fluid very quickly. Once you lose that viscosity, what tends to happen is that the oxidation rates start to increase. When gear oil begins to oxidize, it’s high time to change it.”

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