How to Remove a Lower Unit From a 25HP Mercury

by Will Charpentier

Removing the lower unit of a 25-horsepower Mercury outboard motor is a part of the motor's preventative maintenance program. The lower unit, the part of the motor that encases the gears, the propeller shaft and the cooling water pump, is located at the bottom of the motor, just below the exhaust housing, or "middle" part of the motor. Mercury recommends inspection of the each of these parts and systems in the lower unit once a year or every 100 operating hours.

Rotate the motor upward with the tilt control. Engage the tilt lock pin to prevent the motor from dropping.

Reach under the engine cowling at the front of the powerhead and locate the threaded coupler on the shift shaft and shift rod. Mark the threads of the coupler with a permanent marker, at the jam nut, so you know their exact position for reassembling the shift shaft. Loosen the jam nut and turn the coupler counterclockwise until the shift control rod and the shift shaft separate. There is a flat washer at the top of the shift shaft that you must retain and reinstall when reassembling the shift shaft.

Remove the two bolts threaded upward on each side of the motor, from the upper edge of the lower unit through the lower edge of the exhaust housing, with a 7/8-inch thin-wall socket. Pull out and down on the lower unit while carefully guiding the driveshaft and shift shaft out of the motor.

Items you will need

  • 7/8-inch thin-wall socket
  • Permanent marker


  • "Mercury Marine Outboards 2.5-275HP 1990-2000"; Seloc Marine; 2007

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.