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When did you last replace the zinc pencil inside your engine's heat exchanger? If you don't remember — or worse, if you are thinking "the what?" — this is a maintenance job that needs immediate action. The copper tubes inside the exchanger are at risk of damage from electrolysis — just like your prop. An internal zinc anode protects the exchanger. You will normally find the zinc pencil under a threaded plug near one end of the exchanger. If it is only lightly corroded, scrape it bright and put it back; if it is heavily corroded — or gone altogether — replace it. Check it at least annually.