Don Casey Tip #6  

Zincs

The longevity of a sacrificial zinc anode is a function of its weight ; the amount of protection it provides depends on its surface area . When a zinc lasts less than a year, you need one with more weight. If the protected metal shows signs of corrosion despite the zinc, you need more surface area.

Check all zincs at least annually; if half (or more) of a zinc has been lost to corrosion, replace it. Don't forget to check the zinc pencil inside your engine's heat exchanger; you will find it (or not) under a brass plug in the exchanger.