Mercury Outboard Lower Unit Seals to Replace

To keep the lubricating oil within the gearbox and to prevent water from entering the gearcase, there are a number of seals in an outboard’s lower unit that need to be replaced once in a while. 

There are three shafts on a Mercury outboard engine’s lower unit that have oil seals and O-rings you should inspect and replace when necessary. If you’ve already removed the lower unit for other repairs like a water pump replacement (which has seals of its own), you should go ahead and inspect the seals of the prop shaft, the input shaft and the shift shaft.

Prop (Output) Shaft Seals

If you see oil dripping from beneath the outboard’s propeller, it’s a sign of a leaking prop shaft (or output shaft) seal in the lower unit. 

Because output shaft oil seals are exposed and prone to damage, you should replace them immediately if they’re leaking or worn out. Leaking oil seals at the prop shaft means oil is escaping from the outboard’s lower unit, and is also a sign that water is getting into it, so replacing those seals is a high priority. 

Input Shaft (Driveshaft) Seals

Worn and failing driveshaft (or input shaft) seals lead to oil escaping the gearcase, and water getting in and quickly ruining the outboard’s lower unit. 

Water getting into the gearcase, or oil escaping from it, can be catastrophic, so replacing worn input shaft seals is essential to the health of an outboard. When driveshaft seals start to leak, replace them immediately, before the gearcase suffers major damage. 

Shift Shaft Seals

The shift shaft engages with the transmission to put the outboard into or out of gear. It also interacts with the transmission to dictate whether the prop shaft should rotate in one direction or the other, making the propeller move the boat forward or in reverse.

Seals for the shift shaft are often included with Mercury gear housing kits like the one we used in the video below, which also came with input and output seals for the lower unit, along with a new gasket for the shift shaft housing. If you already have the lower unit removed and you’re replacing one seal, you might as well go ahead and change them all.