Replace An Impeller On A Mercruiser Alpha One

This impeller is critical to the operation of a sterndrive.

The MerCruiser Alpha One Stern Drive is one of the most popular and reliable power plants in use for recreational boating. With proper care and maintenance, the Alpha One can last a lifetime. One of the maintenance items that can be performed by the average boating enthusiast and do-it-yourselfer is replacing the impeller in the lower unit. This device is critical to the operation of the sterndrive, and running the engine with a bad impeller will cause permanent damage.


Remove the Lower Unit

1. Place the gear shift in the forward position and turn the propeller counterclockwise until the unit clicks into forward gear. Remove the propeller and take time to inspect the prop shaft for damage or fishing line that may have wrapped around it.

2. Mark the position of the trim tab. Remove the Allen screw that holds the trim tab in place and remove the trim tab.

3. Beginning at the front of the housing, remove the hex nuts that secure the lower unit to the upper gear housing. Use caution–the lower unit is heavy and will drop free as the nuts are removed.

Replace the Impeller

4. Remove the O-ring from the drive shaft and the water tube guide. Remove the screw and nuts that secure the water pump housing and remove the housing. Using a flat-blade screwdriver, pry the old impeller up and out of the water pump base. Take care not to lose the impeller drive key that fits into the drive shaft.

5. Use a dab of grease to hold the impeller drive key in place on the drive shaft and slide the new impeller down the drive shaft. Be sure to engage the drive key properly. Turn the drive shaft clockwise to get the housing over the impeller.

6. Secure the pump housing with the screw and nuts. Take care not to over-tighten. Install a new O-ring and the water tube guide.

Install the Lower Unit

7. Lift the lower unit into position. Ensure that the splines on the drive shaft and the shift shaft engage properly with their counterparts in the upper gear housing. Start the hex nuts, but do not tighten yet.

8. Start the Allen screw into the trim tab, then tighten the hex nuts a little at a time until the lower unit is secure. Align the trim tab with the marks you made earlier and tighten the Allen screw. Replace the propeller.

9. Supply water to the water intake on the lower unit and start the engine to test for proper water flow.