For the complete engine maintenance story, start with the first article: Periodic Engine Maintenance and Spares.
Many owners are aware of the need of a zinc to protect the prop shaft and strut. But most are not aware of a hidden zinc inside of the engine cooling system. This hidden zinc protects the internal parts from corrosion, particularly the heat exchanger. Both "fresh" and salt water pass through the heat exchanger. The "Fresh" water circuit, like the cooling system in your car, contains a 50/50 mixture of anti-freeze and water. Unlike your car that exchanges the heat in the coolant to the air via the radiator, your auxiliary engine exchanges the heat in the coolant to the cold seawater (raw water) via the heat exchanger. Inside the heat exchanger, hot coolant passes through a series of small tubes. Those tubes are immersed in cold seawater. The heat is absorbed by the seawater and discharged into the exhaust system and then out the transom with the exhaust gasses. The heat exchanger is a horizontal tube mounted behind the engine above the transmission.
The zinc is sacrificial. As the least noble metal in the engine, it corrodes first in saltwater thereby protecting the other metal. Replace the zinc every year. Even if it looks good, don't be a cheapskate. The least noble parts of the zinc are now gone, no longer able to protect your engine. They are inexpensive, easy to replace and protect an engine that costs many thousands of dollars to replace.
We make it easy to order the correct zinc.
Universal was not consistent when it came to zinc size. Different engines used different heat exchangers at different times. Different heat exchangers used different zincs at various times, and your heat exchanger may have been replaced by a previous owner with a new, larger heat exchanger. Replacement zincs are sold by Universal based on the pipe size of the drain plug. But how stupid is that? Even if you were to remove the plug you can't tell its size. For example, there is nowhere on a 1/2" pipe where you can measure 1/2" dimension! But here is how you tell which zinc you need.
The zinc is attached to the winterizing drain plug. The plug is fitted to the bottom of the port end of the heat exchanger. Find the wrench that fits the hex head of the drain plug. Order the zinc you need based on the table below:
- 7/16" wrench order #Z2550
- 9/16" wrench order #Z2551
- 11/16" wrench order #Z2552