Original Factory Mercury Outboard LowerUnits
Your lowerunit is filled with gearlube.
The type of gear lube you use and how often you change
it will largely determine how long your unit will last.
Your engine owners manual will specify the type of lube to use as well
as your lowerunits gearlube capacity and service interval.
Most Mercury Lowerunits use Mercury High Performance GearLube.
Smaller and older units might be able to use Mercury Premium Gearlube but can also
use the tougher High Performance Gear Lube.
Mercury Premium Gearlube is a translucent brown or caramel color.
Mercury High Performance Gear Lube is a translucent greenish color.
You will need a drain pan to catch the old lube and a large broad-bladed slotted
screwdriver to remove the vent screw and the lower fill/drain screw.
Check your owners manual for the capacity of lube you will need and buy a little extra.
You will also need new plug gaskets. Never reuse the plug gaskets or you may leak water into the
lowerunit and cause damage.
You will fill the lowerunit from the lower fill-hole until the lube reaches up to the
side "vent" hole. Mercury makes a neat little fill-pump which fits into the quart sized bottles of gear lube.
This pump works well to fill the lowerunit from the bottom up.
Remove the vent and drain screws and let the unit fully drain. We like to pump a little clean
lube into the top vent hole to flush out the housing. When the unit is fill drained, hook the fill
tube from the pump to the lower drain hole and pump lube into the housing until it flows out the top
side vent hole.
Install the vent plug with a new gasket using caution to make sure to only use ONE gasket and to make sure
that the old gasket is not stuck up inside the hole. Again... always use a new drain and vent plug gasket.
Using a good large slotted screwdriver make sure that both screws are fully tightened.
Check again to make sure both screws are fully tightened and wipe clean to look for leaks.
Take a good look at your gearlube. It's appearance is important.
Water will usually appear as white or "milky". Water will ruin your lowerunit.
If you find water in your gearlube... you need to flush it clean as soon as possible and
you need to find out HOW the water got in.
Remove your prop and check for fishing string wrapped around your propshaft.
If you find string on your propshaft chances are good it has ruined your propshaft seals and the seals will need to
to be replaced.
Replacing the propshaft seals requires special tools and knowledge.
When a bearing starts to fail it will make a metallic "mush" in the gearlube.
If a gear starts to fail you will often find metal bits in the gearlube.
Either of these findings indicate a serious problem.
Simply changing the gear lube will NOT fix the problem.
Do NOT USE the unit until it has been repaired.
You may cause much more damage if you continue to use the unit.
Disassembly, proper inspection and rebuilding a lowerunit requires special tools and knowledge.