Checking and changing the zinc in the heat exchanger on the Universal M25 diesel

Changing the zinc in your heat exchanger is a simple task, right? It kind of depends on how accessible that zinc is. Get your parts on hand and your shop towels to mop up the sea water as it gets onto your nice shiny motor mounts.

Sacrificial zinc: Q: Why do you need zinc in the heat exchanger? A: Galvanic corrosion. The zincs are consumed by any electrolytic action that would otherwise corrode your heat exchanger.

  • Don't wait too long to change or you may end up with zinc chunks in your heat exchanger. If there's nothing on the end of the plug when you remove it, it's been way too long!
  • Everyone's zinc change schedule is different, check once a month until you figure out how often yours needs to be changed.
  • Replace when about half gone.
  • Don't even THINK about putting pipe tape on the end plug threads. The zinc cap pipe threads have to come in contact with the heat exchanger fitting or your zinc is useless.
  • Make sure there's rubber gasket material between your heat exchanger and the bracket it sits on to isolate the HX from the engine.
  • I read about using a ziplock baggie to catch all the water and the zinc on a forum, that works great. Still use a bunch of shop towels, just in case...
  • If you have a replacement end cap you can get just the replacement pencil zinc to screw it into the end cap and save $$.

Parts & Supplies

  • For the Seakamp 302631 CN 3" Heat Exchanger:
    • CME1 - 3/8" NPT end cap, zinc 1/2" diameter by 2" length. $10.49 at West Marine.
  • paper towels and/or shop towels
  • Optional: Gallon size Ziploc freezer bag
pencil zinc
CME1 heat exchanger pencil zinc
socket wrench
Socket wrench and 11/16" socket
pencil zinc
CME1 2" long pencil zinc fits fine in the Seakamp 302631 3" heat exchanger


  • Socket wrench, and for the CME1 end cap - the 11/16" socket.

How to check and change the zinc

Access through the back berth
Access through the back berth
Position your shop towels
Position your shop towels
A little overkill maybe...
  1. Open up the back berth access.
    • Pretty painless to get to the heat exchanger zinc on the Catalina 36 - through the back berth. If you try accessing it through the port side of the engine compartment it takes a lot more contortionist ability.
  2. Get your new zinc handy and your shop towels positioned under the heat exchanger plug.

thru hull closed
Thru hull closed
  1. Close the thru-hull.
  1. With the socket wrench, unscrew the existing heat exchanger zinc end cap several turns.
Loosen with wrench, position baggie, and then unscrew zinc plub by hand - letting it drop into the bag
Baggie Method

Neat and Clean Method #1 - the Baggie:

  1. Position baggie under the plug and then unscrew the plug by hand, letting the plug drop into the baggie.
  2. Catch all the water and any crud in the baggie - neat and clean!

No Baggie? Method #2:

  1. Remove the old plug and zinc, a bunch of salt water will come out. Quickly plug the heat exchanger hole with one of your left thumbs.
  2. With your free hand, grope around for the new zinc with end cap you left behind you instead of where you can grab it easily.

In with the new zinc

  1. Screw in the new zinc with end cap by hand, then tighten with the socket wrench.
  2. Don't forget to open the thru hull valve!
5 month old pencil zinc
5 month old pencil zinc on Reality Check. Could use about 1 more month.
Your zinc change schedule may be different
Pencil zinc change done
Pencil zinc change done
A note on the plug for your helper to do the checking next time

Photos of this project

Checking and changing the heat exchanger pencil zinc

Related topics:
Heat exchanger hose connection diagram
Heat exchanger and overheating - upgrade to the 3 inch and you will not be sorry


Universal M25 Diesel

Heat Exchanger

Engine Compartment Insulation / Soundproofing

Anchor Locker





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Edson Pedestal Controls