By Don Casey
Revised by BoatUS editors in April 2012
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.