How to Replace Pencil Zincs on a Yanmar Engine


The ocean’s salt water constantly dissolves away a marine engine through galvanic corrosion. Engine zincs sacrificially corrode before the engine does, thereby keeping the engine intact. This protection is only effective if the boat owner changes these long, thin pencil zincs regularly. Fresh water-cooled engines use an antifreeze coolant that circulates through the engine. Therefore, these engine blocks do not require zinc installation. Yanmar uses cupronickel in the manufacture of their heat exchangers, negating the need for zincs. Yanmar raw water-cooled engines do use engine block zincs.

Things You'll Need

  • Pliers
  • Metric box wrench set
  • Locate the engine zincs. Consult your owner’s guide, or contact a Yanmar dealer, to determine the exact zinc location on your engine model.

  • Close the engine intake through-hull.

  • Remove the old engine pencil zinc. Unscrew the brass hex-head plug connected to the engine. Use an appropriately sized metric box wrench. This plug contains the old pencil zinc. Unscrew the pencil zinc from the plug with a pair of pliers.

  • Purchase new zincs. Take the plug to a marine supply store. Choose a pencil zinc that matches the plug’s diameter, length and thread. Record the part number for future reference.

  • Install the new zincs. Screw the new pencil zincs into the plugs. Install the plugs into the engine blocks. Tighten with a metric box wrench.

  • Turn on the engine intake through-hull. Start the engine. Run the engine for a few minutes. Shut the engine down. Inspect for leaks.

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