America's Definitive Marine Engine Magazine
Do-It-Yourself:
How To Change
Outboard Motor Gear Oil
Changing your own
outboard or stern drive gear oil saves money, extends an outboard or stern drive lower unit life and provides peace of mind.
It's no secret that changing outboard motor and stern drive lower unit gear oil extends engine life. Being this operation by making sure the outboard or stern drive lower unit is in a vertical/upright position. Drain gear oil a few hours after the outboard motor shut down in order to allow any emulsified water to separate from the oil. Ready to proceed, locate both the upper and lower Fill and Drain plugs. Place an oil drainage pan directly beneath the drive leg bottom drain plug so as to catch the draining oil.
Use long-handled, slot-edge screwdriver (for leverage) to break loose the bottom drain plug. Be patient, don't get in a hurry and unnecessarily burr the edges of the slot. It looks bad and makes you look like an amateur. Just barely crack the lower drain plug screw. Take mental note of what fluid issues forth, gear case oil, a milky froth, or water.
Even a scant few drops of water means you ought to check more often in case the problem gets worse. However, an ounce or more means a seal has failed. Undetected in its early stages, a leaking seal can cost hundreds of dollars to repair. But catch it early and oftentimes only the propeller shaft seals have to be changed, and not the expensive gears, shaft and bearing set. It's rare for the driveshaft or shift rod seals to fail, but it happens. Marine mechanics pinpoint exactly which seal is leaking with a pressure tester pictured in the foto above that pumps air into the gear case
The gear case is dunked in the water. A trail of bubbles escaping from one of the seals means it's leaking. Similarly, after changing seals, it allows checking your work. Don't assume it's the propeller shaft seal that's leaking. It could be a failed driveshaft or shift rod seal located on the top of the lower unit.
When changing lower-unit oil, examine the spent oil for signs of overheating. Dip you finger in the oil, smear it across a finger. Sniff it. The telltale clue of burned oil is an unmistakable, burned smell and very black oil. Milky brown means water mixed in with the oil by the whirling gears. Broken chunks of gear teeth or bronze bits also mean serious mechanical damage. Dunk a magnet in the oil to discover metal bits you can't see with the naked eye.
Patiently wait for all of the spent oil to completely drain from the lower unit housing, a process that takes from 5 to 10 minutes. Once the last drop of oil has trickled out. Before topping off first make sure each gear case drain plug wears one and only one gasket. Old gaskets are infamous for hiding inside the housing and the do-it-yourself mistakenly believes a gaskets is lost. Two gaskets don't seal. Neither will none. Also consider outright replacing the gaskets, they stiffen with age and sometimes lose the ability to seal.
To refill use a hand pump and quart bottle of lubricant instead of oil in a squeeze tube. The hand pump's hose features a fitting that screws into the bottom drain plug hole, preventing spillage. The pump is a one-time purchase that can be used again and again. Conversely, squeeze bottles are clumsy and messy, especially if the lower unit requires more than one squeeze bottle of lubricant. Fill from the bottom orifice, adding oil until lubricant just begins to overflow the top hole. Keep an oil absorbent rag or shop towel handy to dab up errant droplets.
Keeping the hand pump hose attached, or squeeze tube snug against the bottom drain hole, install the top plug. Remove the oil filler (tube or hose) from the bottom hole and install its drain plug. Do so swiftly, thereby minimizing oil loss. Even though slight vacuum holds the charge of oil inside the gear case, inevitably some oil will spill over into the drain pan. Hand tighten both the lower and upper drain plugs and wipe off any oil that may have streaked onto the lower unit.