How to remove the lower unit on a mariner 25hp

Written by will charpentier
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An outboard motor's lower unit is home to the propeller, the water pump and the gears that convert the motor's noise into motion. When you need to remove the lower unit on your Mariner 25-hp outboard motor, few tools are required and you need not remove the propeller or drain the gear case lubricant to complete the task. The Mercury Mariner 25 comes in two versions, the Marathon and the Seapro; mechanically, both versions are identical.

Skill level:

Things you need

  • 5/16-inch combination wrench
  • 7/16-inch socket and socket wrench
  • Bench vice

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  1. 1

    Remove the nut from the negative battery post with a 5/16-inch combination wrench. Lift the cable from the battery and set it aside.

  2. 2

    Move the gear lever into the "Neutral" position. Tilt the motor to the full "Up" position and engage the tilt-lock pin.

  3. 3

    Pivot the shift shaft retainer, located under the engine cowling at the front-bottom of the motor's power head, 1/4-turn clockwise. Remove the retainer to free the shaft.

  4. 4

    Use an 7/16-inch socket and socket wrench to remove the two bolts on either side of the lower unit that are threaded upward from the lower unit to the exhaust housing of the motor.

  5. 5

    Pull down on the lower unit to separate it from the exhaust housing of the motor and guide the driveshaft and shift shaft out of the exhaust housing. Set the lower unit upright in a bench vice.

Tips and warnings

  • There's a small, flat washer that will be pulled from the shift shaft as you remove the lower unit; it will fall helplessly to the floor when you remove the lower unit. Be sure to find and keep it, as it's necessary for reinstallation.
  • Always use a 5/16-inch wrench when removing the nut from the negative post of a marine battery. The positive post uses a 3/16-inch nut. If the lettering or cable colour is obscured by dirt or oil, the 5/16-inch wrench won't fit on the positive post.

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