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time to replace lower unit seals? s/n OT087666? 90hp merc

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  • time to replace lower unit seals? s/n OT087666? 90hp merc

    I've got a 90hp 2 stroke s/n OT087666 Merc. About 18 years old now. I check the lower unit oil frequently and for the first time I saw about a teaspoon of water come out with the oil. It was not mixed in and milky but was separate. I replaced all the oil and went on a boating trip putting on about 30 miles. I'll be checking more frequently for water now but I am wondering if it might be time to just go ahead and replace the lower unit seals. I haven't even looked in the manual yet so I thought I'd ask here as to how difficult that job is? Special tools required? I've done all the work that has ever been required on this motor, not very much but replaced water pumps, fuel pumps, cleaned carbs etc so I feel comfortable doing the work myself. However, if takes special tools to get the seals seated correctly etc. then I will have it done by an expert mech for motors from that era. Its been a great motor that I"m not looking to ever replace. I'ts got about 350hrs on it. 30% salt trips.

    I do remember reading that to assure seals are working properly the lower unit should hold a slight amount of pressure.

  • #2
    Properly sealed the unit should hold both pressure and Vacuum.

    Water, any amount, in the case is a concern. Get the case pressure and vacuum tested. It might just be the gaskets under the Drain/Vent Screws. They should be changed every time they are loosened.

    Mercury used to recommend filling their gearcases to the vent level, then draining about an Ounce of fluid out before putting the plugs back in. This was to create an Air Pocket to allow room for the fluid to expand when it warmed up. Most people including Mechanics ignored creating the Air Pocket part and plugged up the case full. Since oils have little or no compressibility, the when the oil gets warm and expands, it has no place to go, except past a Seal. Then when it cools a vacuum would form and this time Water would make it past the seal.
    Other makes may have provided trapped air chambers in their cases, and Mercury may on their newer cases





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    • #3
      Thinking more about it, I may have created this problem myself. About 3 months ago I dropped the lower unit and inspected the water pump. It looked fine so I reassembled and I did not replace any parts. So I may have damaged an o ring or seal when I put it back together. At the time of the water pump check there was no water in the gear oil.

      Thanks for advice regarding leaving space for oil to expand. I probably overfilled it before being paranoid about not having enough oil in there.

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      • #4
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        The upper DRIVE shaft seal is beneath the water pump. You may have your smoking gun. You can pull your pump again and they are right there, pay attention to the direction they are installed. As Jimmbo stated, get yourself some drain/fill screw gaskets while you are buying parts.

        That engine has 2 vent plugs and the manual says that both are to be pulled, fluid is to be injected from below till solid oil (no bubbles) comes out and install that plug....continuing to add fluid till clear oil comes out the rear (center of the LU) hole and plug. Then quickly remove the injection adapter and plug the drain hole.

        The other seals are the shifter shaft and prop shaft, both of which can be fished out without having to pull the LU apart.....with a little finesse. Be careful in removing the shift shaft seals that you don't pull the shifter shaft (vertically) out of the shifter cam as that normally requires a disassembly of LU to get plugged back in.
        If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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