Outboard: milky lower unit oil

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
I brought the dinghy home and while winterizing the motor drained the lower unit to find milky fluid - surely, the unit leaked and has salt water in it.

I refilled it with new oil while I figure out what to do with it. I have two spare lowers so I'm not without a motor, but I wonder:

  • how to protect and preserve the lower before rebuilding it?
  • what needs rebuilding - prop seal?
It's a 1998 vintage Mercury 15HP two stroke.

All suggestions welcome!

Thanks,

jv
 
Jun 14, 2010
2,081
Robertson & Caine 2017 Leopard 40 CT
Possibly prop shaft seal, possibly just the fill hole screws need new washers. I noticed you’re in Westport- you might ask the guys at FL Tripp. It’s been years since I’ve been there but they seemed like honest guys who know their stuff.
 

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Thanks, guys. Yes, Tripp's is tops, but I do my own work on the outboard. Dave, i have the service manual.

I'm thinking I should by all the seals and rebuild it, but several special tools are required. I made a tool for unscrewing the bearing carrier, but the other tools are not so simple!

Any ideas on what to do until I can tear it down?
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
Until you can tear it down...
Change the fluid as often as needed to keep it clean & also keep the lower unit out of the water whenever you can.

I'm not a Merc guy by experience, but you probably need to change at least 2 seals, one on the horizontal prop shaft & one on the vertical drive shaft. You may also need to change a seal on a shift linkage & maybe change a gasket or two. While you are in there that deep, of course you are going to want to freshen up the water pump. As was already said, you may get away with just a plug gasket or two (if you are real lucky). The vertical drive shaft seal is a classic problem that can be caused when an unskilled mechanic is not careful about doing the water pump.

If you cover the lower unit with soapy/sudsy water, then use a blow gun to put about 3psi of pressure in it from the oil fill hole, you will often be able to see what seal is leaking.

Before you dig in too deep, firmly wiggle the prop up & down then side to side. If you feel movement in the prop shaft, then you may have a spun bearing in the case. That would be a more major undertaking that many would write off as a total loss on a salt water lower unit of that vintage. I've repaired mine before, but I have a pretty good machine shop at my disposal. A spun bearing is usually also accompanied by a metallic sheen or fine sludge in the lower unit oil.

What tools do you think you can't make?
 
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jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Thanks, Jim. Good information. I will check for prop shaft wobble, and I still have the oil I drained, I can look for metal.

I will also pressurize it to check for leak areas.

The tool that's easy to make is the one for unscrewing the bearing carrier - I have one already. The tool that might be tough to make is the drive shaft bearing installation tool - 91-824791.

I can send a copy of the chapter from the manual on the lower if you'd like to take a look! (PM your email address.)

jv
 

capta

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Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
I'd just dump the thing in a tub of diesel to store it until you get around to fixing it.
 

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
So, let's say I'm very lucky, and its just an oil drain or fill plug that leaked. I was thinking I'd flush the unit somehow. I have an electric oil change pump; I could rig it to flush it, first with a degreaser, then a mild acid (to remove salt), then a neutralizer, then alcohol to dry it, then refill with oil - perhaps all the while turning the gears to make sure ever surface is treated.

Thoughts?
 

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
I'd just dump the thing in a tub of diesel to store it until you get around to fixing it.
That's a thought. It fits nicely into a 5 gallon Home Depot bucket, and stays upright with the flanges on the bucket edge. And, I happen to have a five gallon jug of diesel.
 

capta

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Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
That's a thought. It fits nicely into a 5 gallon Home Depot bucket, and stays upright with the flanges on the bucket edge. And, I happen to have a five gallon jug of diesel.
Diesel is basically 4% motor oil and the rest is kerosene, certainly a good cleaner and lubricant.
 
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Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
...
The tool that's easy to make is the one for unscrewing the bearing carrier - I have one already. The tool that might be tough to make is the drive shaft bearing installation tool - 91-824791. ...
When I google that tool number, I find an upgraded version that replaced it - https://leadersrpmshop.com/mercury-...MIiu-t6pzU3gIVBT0MCh0ZRAT4EAQYBCABEgJ7YPD_BwE
I'm thinking that a couple of sockets, a couple of washers, a couple of nuts & a piece of thread rod might be enough to get you what you need. Alternately, the "correct" tool seems to be less than $50. You can probably also find a universal installation tool kit at an auto parts store.

In the past, when I have had salt water intrusion in a lower unit, I fixed the leak, changed the fluid, drove the boat for at least half an hour, then changed the oil again & drove the boat for another half hour. If the fluid looked clean, I called it good. If it still looked questionable, I did another fluid change/run/inspection. in my worst case, it took me 4 changes to get consistently clear fluid. After 3 years, that lower unit is still good (but probably due for another water pump now). While you are doing the flush-out lower unit fluid changes, just use cheap gear oil from Walmart. You can put the expensive stuff back in at the end if you want to. On the older motors, I have had good luck just leaving regular gear oil in the lower units.

When you drop the lower unit, if you see pitting on the vertical drive shaft, then the drive shaft seal will be your most likely source of trouble.
 
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jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Or use one of these. (I bought one a long time ago when I had a motorboat.)
Thank you!

I've been thinking about this a lot, probably too much. I was thinking one should have two gauges: a vacuum gauge and a pressure gauge. The vacuum gauge would be for verifying the seal in the Spring, before launching, without draining the oil. If you can pull a vacuum and it holds, it's sealed. The pressure gauge would be for diagnostic purposes - so you can pressurize the lower, after draining the oil, so you can spray it or submerge it to look for leaks (bubbles).

I hope I saved the drain plug screws from my blown lower from 10 years ago. That one had leaked, and then seized form rust over the winter, since I neglected to drain the lower winterizing it. Ugh! I thought I would destroy it taking it apart. I eventually had to use an air impact wrench on the bearing carrier tool. BTW, I can't find that tool that I made. :( I will use one of the drain plug screws to make a gauge adapter.
 

jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
When I google that tool number, I find an upgraded version that replaced it - https://leadersrpmshop.com/mercury-...MIiu-t6pzU3gIVBT0MCh0ZRAT4EAQYBCABEgJ7YPD_BwE
I'm thinking that a couple of sockets, a couple of washers, a couple of nuts & a piece of thread rod might be enough to get you what you need. Alternately, the "correct" tool seems to be less than $50. You can probably also find a universal installation tool kit at an auto parts store.
Thanks again, Jim. The pic doesn't look like the illustration in the manual, though. But perhaps not so hard to make one.
 

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Picked up another good lower for $100! I now have two motors, three good lowers, and one rebuildable lower (the one that leaked).

The plan is to put both of these motors into top shape this winter. I'll start making a list of parts I need, along with evaluating which things need be replaced, and which can be refurbished.

Should I be considering anything regarding the power head, like perhaps reed valves?

I also recently got an ultrasonic cleaner (for my SCUBA regulator work), so I can really clean the carbs.

About the lowers:

I think renewal of all the seals, plus a water pump impeller is in order as a minimum, for unknown lowers (the water pump is part of the lower unit assembly). Then, pull vacuum on it and see if it is sealed. If not, pressurize it in a bucket to find the leak; all of this before adding oil.

This means I'll need to re-make that bearing carrier wrench that I made and can't find (drat!). I think I can check the bearings, etc., and if good not pull/replace them. If I need to replace them I can think about making those tools, too. (I know, I'm cheap! But I can't stand paying hundreds for what are fairly simple special tools.)

Any and all input is welcome.
 

jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Continuing on this - I find the forum a good place to think out loud, figure out what I'm going to do.

To completely rebuild these lowers, with new seals, pump kits, bearing, bushings, etc., including the special tools required, would likely exceed the cost of a new unit, which is about $1,000. So, a strategy is necessary; I'm thinking that I'll only replace gears, bushings, bearings, shafts, pump parts, etc., as absolutely necessary. Impeller, seals, O-rings, and gaskets will be replaced. Shafts that are pitted - will be trashed or welded up and turned on the lathe. That is, pitting in the seal area is a problem, but pitting can be welded up and then the shaft turned and polished on the lathe to restore the sealing surface. This only because a prop shaft and drive shaft are $250 and $180, respectively, and I happen to have a TIG welder and lathe!

I am thinking that as these age, vacuum testing at the beginning, and perhaps mid-season is key, to catch a leak in time.
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
...Shafts that are pitted - will be trashed or welded up and turned on the lathe. That is, pitting in the seal area is a problem, but pitting can be welded up and then the shaft turned and polished on the lathe to restore the sealing surface. This only because a prop shaft and drive shaft are $250 and $180, respectively, and I happen to have a TIG welder and lathe!
...
If that shaft is heat treated (probably likely) then welding may not be your best option for pit repairs.