How to Change the Water Pump on an OMC Sterndrive

by Will Charpentier

The maintenance specifications for your motor and OMC sterndrive are specific: Check the impeller of your sterndrive's water pump yearly, or after every 200 hours of operation. Since you have to disassemble the sterndrive and water pump to inspect the impeller, you might as well change the pump while you have the sterndrive apart. Changing the pump is a one-person job. Removing the sterndrive is less uncomfortable -- because of the its weight -- when done by two people.

Removing the Drive and Changing the Pump

1. Wrap a blanket around the jaws of a bench vise, to serve as a temporary stand for the sterndrive, while you change the water pump. Open the vise to its fullest extent, until you set the drive in place on the bench.

2. Shift the motor into "Forward." Remove the decorative plastic caps. Remove the lock nuts with a 5/8-inch socket.

3. Remove the two small washers, the four rubber bushings, the two large washers and the anchor pin from the aft end of the the hydraulic trim cylinders. Pull the piston end of the cylinder from the lugs on the sterndrive, by hand.

4. Remove the six lock nuts and washers that secure the sterndrive to the bell housing, using a 7/8-inch socket. Pull the sterndrive straight away from the bell housing. Move the sternddrive to the bench vise -- an assistant's help is recommended during the transfer -- and tighten the vice only enough to hold the sterndrive upright.

5. Loosen the trim tab screw, using a screwdriver, and remove the trim tab from the drive. Remove the Allan screw found under the tab. Remove the two nuts beneath the drive's ventilation plate with a 5/8-inch socket.

6. Remove the two nuts on each side of the drive that hold the upper and lower housings of the drive together. Lift the upper housing away from the lower housing.

7. Remove the pump housing nuts from the pump housing with a 1/4-inch socket. Lift the housing from the shaft. Remove the impeller, key and wear plate from the shaft. Slide two slotted screwdrivers under the sides of the water pump base. Pry straight up to remove the base.

8. Slide a new shaft seal down the driveshaft and press it into the large hole in the top of the gear case. Spread a thin layer of gasket sealant on the bottom of the the new water pump base plate, from the pump repair kit, and slide the plate down the driveshaft so the sealant contacts the top of the gear case.

9. Grease the center hole and the outer edges of the fins of the impeller with waterproof marine grease. Slide the wear plate down the shaft, aligning the holes on its corners with the bores for the pump housing bolts. Slide the impeller down the driveshaft and onto the wear plate.

10. Dab your finger into gasket sealer and run it around the bottom edge of the pump housing. Press the second driveshaft seal into the larger of the two bores on the top of the pump housing. Squirt a small amount of white marine grease into the center of the seal to help you ease the housing and seal down the driveshaft.

11. Place a bit -- just a thin smear -- of thread locker on the threads of the pump housing bolts. Push them down through the bores at the corners of the pump housing and into the top of the gear case. Tighten them as much as possible with your fingers, then tighten them to 55 foot-pounds with a torque wrench.

Reinstalling the Sterndrive

1. Set the upper sterndrive housing onto the lower stern drive housing. Replace the two lock nuts on each side of the drive that secure the upper housing to the lower housing.

2. Replace the Allan screw beneath the ventilation plate and replace the trim tab screw. Install the trim tab in its location below the ventilation plate and secure with the trim tab screw.

3. Lift the reassembled stern drive from its temporary work stand -- the assistance of a friend is highly recommended -- and push it straight back into the bell housing.

4. Place the six mounting washers on the six studs around the bell housing and thread the lock nuts back into place around the gear case.

5. Pull the piston-end of the hydraulic trim cylinders onto their lugs on the sterndrive. Replace the anchor pin in the lug, push the two large washers into place along with the four rubber bushings, the two small washers, the lock nuts and plastic trim caps.

Items you will need

  • Blanket
  • Bench vise
  • 7/8-inch socket
  • 2 screwdrivers
  • 5/8-inch socket
  • 1/4-inch socket
  • Pump replacement kit
  • Gasket sealant
  • Waterproof marine grease
  • Thread locker
  • Torque wrench
  • 5/16-inch box-end wrench


  • Disconnect the negative cable of your battery before performing any maintenance work on your sterndrive or engine to prevent electrical shock or accidental starting. Remove the nut from the negative post with a 5/16-inch box-end wrench. Lift the cable from your battery, move it outside of the battery box and close the lid of the battery box. After the work is complete, reconnect the negative battery cable.


  • "Mercruiser Service Manual Number 11 - Bravo Sterndrives"; Mercury Marine; 1998

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.