Mercury 500 lower unit leak??

I just replaced the threaded nut that screws into the lower unit just in front of the propellor. I tightened it just like the guy at the parts place said. It's leaking all the oil I put on the lower right back out. Not super fast but either way any leak is bad. Should I replace the giant o-ring?? What do I do?

Guessing with the info supplied. Your talking about an older 4 cyl engine that has a large spanner nut, almost the diameter of the LU housing, holding the rear bearing carrier in place and the oring mentioned is the one imbedded in that carrier? Apparently you have some corrosion that became dislodged, making a leak path, or you didn't get the oring set right. Did you lube up both the oring and the housing before sliding in the R gear and carrier? What are you using for a spanner wrench?

Mark

You need to pull the prop shaft assembly out and replace not only that big O-ring, but the shaft seal as well. Both can be purchased locally at a bearing house--just take the old parts in for them to match up.

Jeff

Okaye, in re-reading this you didn't do anything with the bearing carrier assembly per se. Your problem was just replacing the spanner nut. So I agree with Jeff you disturbed the seals and while you have it out might as well do both, actually some engines have 2 prop shaft seals. If so, one is designed to seal against the LU oil the other the environment. Therefore the lips will be reversed; one pointing out the other in. The Merc service manual for my engine shows the seal(s) installed with Locktite 271 which is the red Locktite.

But one thing is confusing. Why were you fooling with that nut in the first place if it wasn't to replace the seal?

Mark

Thanks for all the info guys and I just bought the boat. Got it home and could slide the shaft and propellor right out of the lower unit when it got to my house. Local shop says that water must have gotten into it over the winter and froze inside forcing the spanner nut to pop out?

Doesn't sound good. You could take it to a shop and have them pressure test it for you or just get the vent or drain screw and take it to a hardware store and have them work you up an adapter to a Schrader valve (the air valve in tires). Then you have an interface to readily available air but watch the 8 lbs. You can get there in a hurry with most air supplies.

On the flip side, if in fact the housing did split, as indicated by a visual of the crack or the bubble pressure test, when you pull the bearing carrier, R gear drive shaft, clutch dog and shifter cam follower (all one assy) out, a prop shop could weld the crack for you as there is nothing left down there, other than fill drain gaskets on the screws, to be damaged by the heat. Replace your gaskets as mentioned earlier, including new drain and vent screw seals....Merc dealer has them. Put a little vaseline on the big oring; not a lot, just enough to make assy smoother.....don't want to get it in your threads....see next sentence.

Clean up the threads with a thread chaser (a machine shop supply house will know what that is), degrease adequately and after installing the drive assy and all, install the spanner nut using blue locktite....auto stores has it. It's a thread locker.
Pressure test again to ensure you have a tight fit and refill with marine lower unit oil....WW has it in the sporting goods dept along with TC-W3 rated 2 cycle oil for the gas.

Mark

ok guys again I want to say thanks for all the info but I did find my problem and it was a complete noob move for anything requiring mechanical know-how. I didn't remove the broken pieces of thread from the old nylon spanner nut, therefore the new spanner nut felt tight but wasn't even close yet. Problem has since been resolved after I replaced the seal and o-ring and put the "key" in that was missing when I removed the carrier. Thanks again for all the help guys.

Geez, another success story. "Whadda deal". Great. Glad you got her fixed.

Mark