Outboard Lower Unit

M

mjsiega

New member
I just changed the lower gear oil in my 75HP outboard and noticed that it was somewhat milky in color. I didn't notice any metal parts (fine or otherwise). Is it normal to have a little water in the lower unit? The owners manual states that it's ok to see fine metal in the oil, but mine came out without any metal at all, which is leading me to believe this is ok...? Thanks.

-Mark
 
Milky oil isn't a good thing. It is probably a the seal on the prop shaft. But there are other ways for the water to get in. The way they test for this is to put a small amount of air pressure on the gear case like 1 or 2 lbs and see if it will leak out. A lot of the time it is cause by fishing line wrapped around the prop shaft ruining the seal. If your not comfortable with doing this I would take it to a dealer. They will have the proper tools and test equipment for this.
 
My original routine was to check my oil at 50 hours by briefly loosening the lower plug. It was ALWAYS ok. I'd go to change it at 100 and it was ALWAYS milky. I had my tech run the prescribed OMC test (vacuum or pressure, I don't recall). It was fine.

Long story short, by loosening the lower plug I'd compromised it's nylon washer/gasket.

Now I simply replace my gear oil (and both washers/gaskets) every 50 hours.

The moral of this story...those nylon washers/gaskets are good for ONE tightening only.
 
I have to echo the previous post. After checking my gear oil and finding some milkyness to it, I conducted the air pressure test and confirmed the seals were good, I went back to the nylon washers on the drain screw. They are good for only ONE tightening.
I re-checked the oil this summer (changed oil last winter with new washers), put screw back in without replacing washer and after about 10 hours of use, got some water infiltration. It has to be those white nylon washers. My plan now is to replace them every time I change the oil, every season or 50 hours of use just to be sure.

Grazer
 
Milky oil is not good and does indicate a problem, but a marine mechanic told me once that he had never seen a lower unit overheat as long as it was full and that water emulsified oil will still lubricate. This is not to say that it will not cause problems or that it should not be fixed.

The 1980 50hp. Merc on my Thundercraft had this problem a few years back - the mechanic found that the drive shaft was badly pitted around where it passed through the water pump housing. A new shaft was no longer available, so he removed it and gave it to me. A guy that I work with(from the Azores where they are accustomed to fixing things instead of replacing) cleaned the shaft and filled in the pitted area with brass then had a machine shop turn it down to the original dia. The marine tech then reinstalled it along with a new water pump and pump housing(which was also worn) and I am still using the boat years later.

Fishing line can be a problem - I tore the seal out of the lower unit on my Campion years ago. I don't know what all applications they are available for but they do have line cutters that can be installed behind the prop on some units. The 1974 Merc 6hp. that I used as a kicker on the Campion had one of these - it was designed to cut fishing line before it could get wrapped around the seal area.

Regards, Rob
 
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