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Q&A - Pencil zinc

Q: How often should you check and change the pencil zinc in your inboard engine’s heat exchanger?

Q: How often should you check and change the pencil zinc in your inboard engine’s heat exchanger?

A: Your pencil zinc will deteriorate at different rates under varying circumstances. It is critical that you not let it completely erode. It should always be at least partially intact — the more the better.

The purpose of the pencil zinc is to help prevent electrolysis from eating away the metal in your heat exchanger and engine components that are exposed to the raw water. This could cause such problems as holes in the stack of tubes, which will allow raw water and the engine’s cooling fresh water (with antifreeze) to mix; degradation of component sealing lips; or holes in the body of the heat exchanger, resulting in leaks into the engine space. Replacement of these components can run into the thousands of dollars, while pencil zincs typically cost only a few bucks.

Here are a few things that increase the rate of zinc deterioration:

1. How much you run your engine. Water racing by the zinc helps it erode, in addition to the erosion caused by electrolysis.

2. The amount of silt in the water in which you run.

3. The salinity of the water in which you run.

4. The metals within your cooling system that are exposed to raw water. Some systems are more vulnerable to electrolysis because the alloys are at different levels of separation on the nobility scale.

5. Electrical events within your boat — for example, even a small current leak to the engine block or surrounding water.

6. Electrical events outside your boat — for example, even a small current leak from nearby boats or dock wiring.

Obviously, the more often you check the pencil zinc the better. Eventually you will notice a pattern of erosion upon which you can rely somewhat, but remember this can change.

Be sure to close the engine raw water intake seacock (and any other relevant seacocks) before you unscrew the zinc. Take all other precautions relevant to the job and circumstances, such as making sure there is no danger from electric shock as you touch and work around the engine with wet hands. The engine should be cool.

Drain the raw water side of the cooling system so that water from the zinc cavity won’t spill out on engine components — alternator, starter, etc. — when you unscrew the zinc holder. Don’t start the engine unless the raw-water pump is again flooded or you’ve greased it to protect the impeller. Check inside the zinc hole with a small, protected, waterproof flashlight for debris, as from the remainder of the zinc. Sometimes this will accumulate to the extent that it can impair water flow; you should remove it if this happens. Small amounts of zinc debris are normal and will often break apart and wash through.

When replacing the pencil zinc don’t overtighten it, which could split the metal around the threaded port. And don’t use Teflon tape or other sealant, as this could impair the conductivity between the zinc holder and the heat exchanger. After running, check that it isn’t leaking. I keep at least six pencil zincs for each application on board.

You can buy pencil zincs attached to the threaded plug or just the zincs that you screw into the threaded plug. For the latter, you’ll have to clean out the stub of the old zinc from the plug. This sometimes can be done with a sharp tool, such as a dentist’s pick, though the job can be rather difficult and might require a drill. You’ll then have to sufficiently tighten the new zinc inside the plug so that it won’t work out due to flowing water and vibration.

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