How to Remove a Lower Unit Mercruiser

by Norm Rogers

The proper way to remove the lower unit on a MerCruiser stern drive is completed in two steps. First, you remove the sterndrive and then you separate the lower unit from the upper gear box. It is possible to drop the lower unit while the stern drive is still attached to the boat, but replacing it is very difficult. In addition, removing the stern drive allows for inspection of the u-joints and other important components.

Remove the Sterndrive

Lower the sterndrive completely and drain the gear oil by removing the lower fill plug and the upper vent screw. They are found on the starboard side of the drive unit. Check the lubricant for excess metal filings or water. Metal filings or water are signs of trouble in the lower unit. If these are found, you may want to have the drive inspected by a professional mechanic.

Disconnect the trim cylinders by removing the 9/16 inch nuts on the cylinder anchor pins. These are the pins that attach the cylinder to the lower unit. Use caution when removing the trim cylinders, as it will cause the sterndrive to drop freely. Take care to prop it up before taking this action.

Remove the rubber bushings and pull the trim cylinder out from the anchor pin. Make sure to retrieve the spiral spring for later replacement.

Put the shift selector in forward gear and remove the six nuts that hold the sterndrive to the bell housing. Remove the sterndrive by pulling it straight back.

Lower Unit Removal

The best way to remove the lower unit is to safely secure the drive in a holding fixture. Keep in mind, you will be lifting the upper gear housing up and away from the lower unit. Before separating the unit, mark the location of the trim tab to make sure you get it back in the right place.

Remove the Allen screw above the trim tab and remove the trim tab.

Remove the 5/8 inch nuts from the anti-cavitation plate, the leading edge of the drive shaft housing, and the sides of the housing. Lift the upper gear housing away from the lower unit. The task is complete

Items you will need

  • Socket set and socket wrench
  • Locking pliers or vice grips
  • Allen wrench set
  • Sterndrive mounting fixture

Warning

  • Take care when removing a sterndrive or lower unit. These parts are heavy and can cause injury if handled incorrectly.

About the Author

Norm Rogers is an experienced writer, publisher and blogger. His works range from formal business documents to books and articles on travel and leisure. He now make the Twin Cities his home.