How to check the lower unit gear oil on a mercruiser alpha one outdrive

Written by chris stevenson
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How to check the lower unit gear oil on a mercruiser alpha one outdrive
The Mercruiser Alpha One engine is typically found on stern-drive models. (Jupiterimages/ Images)

The lower unit gear case oil on the Mercruiser Alpha One outdrive serves a very important function. Proper oil level and quality insures that the lower end transmission gears and shafts have enough lubrication to spin freely and smoothly, with no friction. Lower unit gear case oil also provides a cooling effect so the components do not overheat. Every boat owner should check the level and the condition of his lower unit gear case oil, as often as prescribed in the manufacturer's's service manual.

Skill level:
Moderately Easy

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Things you need

  • Owner's service manual
  • Socket set
  • Ratchet wrench
  • Screwdrivers
  • Drain pan
  • Scrap paper
  • Gear case oil (if applicable)
  • Hand pump (if applicable)

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  1. 1

    Remove your boat from the water and place it on a trailer, if you have it moored or docked. Transport it to a convenient work location. Before you check the lower unit gear oil, make sure the boat trailer does not sit on an incline -- the surface must be flat and level. Adjust your motor position so that it sits straight up and down -- you cannot check the gear oil level with the engine trimmed up or down, or locked in the upright position.

  2. 2

    Place a drain pan directly under the lower unit. Locate the top vent hole oil plug on the lower unit side case. Refer to your owner's service manual for its location, if you have trouble finding it. The vent hole plug will be high up on the lower unit case and marked accordingly. The screw plug will sit countersunk and have a large slot screw head. Use a large flat screwdriver to turn the screw counterclockwise and remove the plug.

  3. 3

    Watch the vent hole after you remove the vent plug. If the oil level sits at the proper level, oil will begin to seep out of the hole and dribble down the lower unit case. This indicates a full level, so you can replace the vent plug and tighten it with a screwdriver. If you see no overflow from the vent hole, stick your pinky finger down into the hole and pull it out.

  4. 4

    Use a curled up piece of scrap paper, bent at a 90-degree angle, to check in the same fashion. No oil on your finger, or oil that sits below the vent hole opening, means you need to add lower gear case oil. In the case of a full or low oil level, examine the oil for a tan or creamlike colour and frothy appearance. Lower gear case oil that resembles the colour and consistency of a milkshake indicates water contamination. Water in the oil points to a bad seal or gasket.

  5. 5

    Rub the oil between your fingers and feel for grit. Look at a smear of the oil in the sunlight and notice if it has any shiny metal reflections. Metal slivers in the oil indicate metal bearing shavings. Change the oil if it shows any of these symptoms.

Tips and warnings

  • When adding oil to a MerCruiser Alpha One motor, you must unscrew the lower gear case oil plug, which is down near the propeller. Quickly attach a hand pump hose (attached to oil bottle) and pump oil into the drain plug until it overflows out of the vent hole plug. Apply several extra pumps, then quickly reinsert the vent and drain plugs and tighten them with a slotted screwdriver.
  • Never add or change the lower gear case oil from the vent plug! Serious damage to the gears can result from air pockets.

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