How to Change the Water Pump on a Yamaha 50-Horsepower Outboard

by Will Charpentier
The water pump is in the motor's lower unit, with the propeller shaft.

The water pump is in the motor's lower unit, with the propeller shaft.

When you do winter maintenance on the Yamaha 50-horsepower outboard motor after each boating season, you should change the water pump. The pump supplies the water that cools the entire engine. Failure of the pump would mean the engine would overheat in a matter of minutes and suffer massive internal damage. All of the parts you'll need for the job are found in the pump replacement kit, and the tools are those found in any boater's tool kit.

Removal

1. Remove all four of the offset water pump bolts and their washers from the lower unit, using a 1/2-inch socket.

2. Remove the pump cover and gasket from the shaft, discarding the gasket. Remove the pump impeller and fiber washer from the driveshaft and pull the Woodruff key from its slot on the driveshaft. Pull the lower fiber washer from the driveshaft.

3. Inspect the face plate of the pump visually to determine if it needs replacement, before you remove it from the top of the lower unit. Look for warping, scratches large enough to catch a fingernail when you run the tip of your finger over the plate, changes in color and any other signs of signs of excessive wear. Remove the face plate, if necessary, and discard the face plate gasket.

Installation

1. Inspect the top of the lower unit, if you must replace the pump’s face plate. Lower the a gasket into place and follow it with the face plate -- or the new face plate -- if necessary. Note that the face plate and gasket are offset, so they will only fit on the lower unit's upper face one way.

2. Set a new lower washer into place and place a touch of marine grease on the back of the Woodruff key. Place the key into its slot on the driveshaft. Slide the new impeller into place so that its drive keyway is aligned with the Woodruff key. Lower the upper fiber washer onto the impeller. Pop a new O-ring in the pump cover. Coat the O-ring and the inner side of the pump cover with a Teflon-enhanced marine grease.

3. Fit the pump cover over the end of the driveshaft. Nudge it downward with one hand while turning the driveshaft to the right -- clockwise -- until the cover settles into place on the face plate. Apply a bead of thread locker to the threads of the water pump bolts. Thread the bolts into place and tighten to 50 inch-pounds with a torque driver.

Items you will need

  • 1/2-inch socket
  • Marine grease
  • Teflon-enhanced marine grease
  • Thread locker
  • Torque driver

References

  • “Yamaha Outboards Repair Manual, 2.5- to 250-Horsepower, Two-Stroke, 1997-1999”; 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.

Photo Credits

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