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How Do I Install the Water Pump on a 7.5 HP Mercury Motor?

Updated February 21, 2017

Installing a water pump on your 7.5 HP Mercury outboard motor is a straightforward process best undertaken as part of inspection or other maintenance of your motor's lower unit. It may also become part of your preparations for long-term storage during the off season. The pump is located directly atop the motor's lower unit and the pump's impeller is driven directly by the motor's driveshaft. Organising the parts in the order you install them simplifies the process.

Set the motor's lower unit upright in a bench vice. Slide the gasket face plate and the lower fibre washer, both from the water pump rebuild kit, downward on the driveshaft to the water pump's base plate. Lightly touch white marine grade grease onto the Woodruff key which you removed from the driveshaft with the old water pump, to keep it in place in the keyway on the driveshaft.

Align the keyway in the impeller with the Woodruff key on the driveshaft and slide the impeller down the driveshaft. Press the impeller into place with the keyway aligned. Slip the upper fibre washer onto the driveshaft and slide it down on top of the impeller.

Lubricate the inner pump surface and the new O-ring with marine grease and fit the O-ring to the bottom of the pump's cover. Slide the cover onto and down the driveshaft to the impeller. Press down on the pump cover while rotating the driveshaft clockwise to seat the cover on the plate.

Apply a thread-locker to the threads on the pump's four bolts and thread the bolts into place. Torque the bolts to 50 pound-inches.

Apply white marine grease to a new water pump tube seal from the pump kit and push the seal into the recess in the water pump cover. Slide the water tube guide over the seal.

Tip

The water pump screw holes are offset. The holes in the gasket and plate only line up one way. Never reuse an impeller.

Things You'll Need

  • Water pump rebuild kit
  • Bench vice
  • White marine grease
  • Torque wrench
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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.