How to Replace Zinc Anodes in a Powerhead for a 90 HP Yamaha 2-Stroke

Changing out the zinc anodes on your two-stroke, 90-horsepower Yamaha outboard's powerhead presents little difficulty, even if you've never changed one before. There is only one anode, and it is always on the same side of the powerhead. Once you remove the engine's top cover and expose the powerhead, it requires the most basic hand-tools to replace it -- a wrench and a screwdriver. With a digital multimeter, you can test the electrical connectivity of the zinc, as well as that of every zinc on the boat.

Things You'll Need

  • Screwdriver
  • Adjustable wrench
  • Digital multimeter
  • 5/16-inch box-end wrench
  • Propeller wrench
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Instructions

    • 1

      Push in on the center of the latch at the rear of the motor's top cover and pull the cover straight up to remove it and expose the powerhead.

    • 2

      Move to the starboard side of the engine. The zinc anode is halfway up the powerhead on a boss -- a metal standoff -- on the side of the engine. Compare the anode to a new anode intended for the same location.

    • 3

      Remove the nut and washer from the bolt that holds the anode to the boss, using an adjustable wrench to turn the nut and a screwdriver to hold the bolt, if the old anode is two-thirds the size of the new anode. Remove the bolt and the anode from the powerhead.

    • 4

      Turn a digital multimeter on and move its control knob to the setting for resistance, ohms, and place one probe against the newly mounted anode. Place the other probe against an unpainted, uncorroded location on the motor, such as the cylinder head, and read the multimeter. If the resistance is very high or infinite, the anode is insulated -- by paint, wax or marine growth, like algae -- and won't protect your boat properly.

    • 5

      Remove the old anode and scrub the surface of the boss where the new anode will sit to ensure a good electrical bond between the engine and the anode.

    • 6

      Visually inspect all zinc anodes on the motor. One is located on the gear case -- in some models, the trim tab on the bottom of the cavitation plate is the anode. In others, the anode is located on above the cavitation plate on the side of the exhaust tunnel, or on the bottom of the gear case.

    • 7

      Repeat the multimeter test on any zinc that doesn't appear worn, without removing the anode. Compare the size of all the anodes on the motor, even those you don't replace, to anodes designed for the same application.