How to Change Lower Unit (Gearcase) and Sterndrive Oil

Lower the outboard or sterndrive so it is vertical, with the drain screw at its lowest point. Place a clean pan underneath. A clean pan matters: you want to inspect what comes out.

The drain/fill screw is near the bottom of the gearcase, often just above the skeg. The vent screw is higher up, usually just above the anti-ventilation plate. Some gearcases have more than one vent screw, and on some models you must remove the prop to reach the drain. Check your manual.

Do not confuse these with the screws that hold the gearcase together or the trim tab bolt.

Loosen the bottom drain screw first and pull it out. Then remove the top vent screw; air coming in lets the oil flow freely. Let it drain completely.

Most drain screws have a magnet. A fine gray paste on it is normal wear. Metal chips or slivers mean internal damage and a dealer inspection.

Look at the color and feel:

• Amber or dark, clear: normal
• Milky, creamy or gray: water is getting in, usually past the prop shaft or drive shaft seals
• Water drips out first, before the oil: definite water intrusion
• Burnt smell or very dark: overheated lube; something may be wearing

Not sure? Pour some into a glass jar and let it sit for a few hours. Water settles to the bottom as a separate layer.

Two false alarms, according to Mercury: lube from a gearcase that just ran can look yellowish from churned-in air, and lube mixed with assembly grease (common after a dealer replaced seals or rebuilt the gearcase) can look creamy white without any water in it. The jar test tells you which it is.

Water in the gearcase will rust gears and bearings and can freeze and crack the case. Refill with fresh lube so the parts are protected, but have a dealer pressure-test the gearcase and replace the leaking seals soon. Fishing line wrapped around the prop shaft is a common cause.

If you removed the prop, look behind the thrust washer for line wrapped near the seal. Line often hides in the old grease, so wipe the shaft and work it out carefully with a pick. If you pulled line out of the seal area, or it looks melted, treat it as a warning sign: look extra hard at the old lube for water. Grease the shaft before reinstalling the prop to the torque in your manual.

Remove the old gaskets or crush washers from both screws (use a pick if they stick to the gearcase). Install new ones. Reused gaskets are a common source of water leaks.

Thread the pump fitting into the bottom drain hole. Pump lube in until it flows out of the top vent hole with no air bubbles. Always fill from the bottom up; filling from the top traps air and leaves the case low.

Wait about 5 minutes for trapped air to rise, then pump in a little more until lube flows from the vent again.

With the pump still attached, install the vent screw first. This creates a vacuum that keeps lube from rushing out the bottom. Then remove the pump and quickly install the drain screw. Tighten both to the spec in your manual; they are small and strip easily.

On sterndrives with a gear lube monitor bottle in the engine compartment, fill it to the line marked on the bottle.

Wipe the gearcase clean so any new leak will be obvious. After your next run, check for drips around the screws and prop shaft. Log the date and hours. Recycle the old lube with your used motor oil.