Outboard Lower Unit Seals Replacement


Six Pointer
How hard of a job is this? Any special tools needed?

I bought a 2006 Mercury 25hp EFI 4 stroke outboard that only had about 25 hrs on it. Been started occasionally but not in the water for a long time. Went thru it with regular routine maintenance, changed engine oil and filter, fuel filter, changed lower unit gear oil. Installed new fuel line from tank to motor connection. Took it out for a couple hours and when I parked it at home that evening, the next morning I found a half-dollar sized puddle of fresh gear oil under the prop on the floor. I suspect where it has sat in a barn so long that the seals behind the prop (not sure what they are called) are probably bad from being dry so long. Upon inspection the gear oil is oozing out from behind the prop.

How big/hard of a job is it to replace these seals? I'm a DIY'er for most things but if it's over my head or requires a bunch of specialty tools, I can take it to a qualified marine mechanic.....just hate paying labor rates for something I can do myself. Thanks for any advice in advance.


Old Mossy Horns
Not a bad job but if the motor has been in salt water can turn into a bitch. I would suggest an OEM service manual if you are going to keep it and work on it.


Twelve Pointer
If it is the seal behind the prop, it isn't a hard job. Drain the lower unit first.

If you don't have a seal puller, there is another method of pulling the seal that I have used successfully in a number of different applications.

I use a small drill bit to drill into the seal. Make sure not to drill into the housing but only into the flat edge of the seal. Screw a screw into the hole that you can grab with vice grips. Tap the vice grips with a hammer ant the seal should pop out easily. If the screw pulls out, start the process over.
Be sure to lubricate the new seal and be sure it is pressed in smoothly and evenly until it hits the seat point.


Six Pointer
Thanks for the advice. I watched a couple vids of the process, and believe I could do it myself...if...I had a couple specialty tools like a seal puller, which I dont and hate to buy one for possibly a one-time use. So, I ended up taking it to a Mercury dealer to get it fixed.