Shop & Field: Broken Zinc

This may seem too boat-specific to have broader value, but it might  apply to anyone who uses threaded pipe fittings.

Sometimes, you happen upon solutions to commonly encountered problems, and you just wish somebody was around to witness your clever resourcefulness and raise a little cheer.  Then again, you wish a lot of things.

Zincs, as most of you know, are used in marine applications to protect valuable metal stuff from corrosion: an inexpensive bit of zinc is placed in contact with bronze (or other more ‘noble’ metal) fittings, and the zinc will sacrificially corrode to nothingness before the costly fitting is damaged by the unyielding chemical process of electrolysis.

One common style of zinc is a rod shaped bit that is threaded into the underside of a specially prepared brass pipe plug.  This plug is then threaded into whatever needs protection, typically a bronze pipe, pump, or in this case, an engine cooling component.  The component, in this case a heat exchanger, has a steady flow of seawater through it.  A ‘pencil zinc’ assembly is threaded in so the zinc simultaneously contacts both the bronze of the heat exchanger and the seawater, the insidious agent of the electrolysis process.

By design, these zincs disappear over time, leaving only the brass pipe plug. So, time for a new zinc- but, Zounds!, the brass plug has simply refused to come out- it’s head twisting off with disturbing ease.  Oh well, just get a new $2500 heat exchanger, and spend another $1000 replacing it- or, remove the stubborn remains of the broken fitting using this trick.  (A more commonly encountered non-marine situation might be found under the kitchen sink, on the boiler in the basement or elsewhere in the world of plumbing.)

The photos show the cramped engine room of a small yacht.  The cylindrical thing is the heat exchanger;  the hex-shaped lug on its the bottom is threaded to receive the brass plug/zinc assembly.  In the first two photos (the starboard engine), the square end of the brass plug, has cracked, but has not yet twisted off.

Both hands were occupied, so I don’t have any photos actually showing the reaming operation, but it works like this: Using a construction reamer in a cordless drill, it was simply a matter of gently reaming into the broken off fitting, taking care to keep everything straight, and stopping frequently to inspect the progress.  National Pipe Thread has a very gentle taper, and the geometry of the reamer comes very close to matching the taper at a certain point along its length.  With care, I was able to ream away all of the broken brass pipe plug except for the last fraction of its threads; the bronze female thread of the heat exchanger was untouched, to the best of my ability to observe it.  Certainly, there was no functional damage.

The photos with the brass curl show the port engine after reaming.  The curl is that remaining fraction of the thread of the brass plug, which unwound easily out of the hole using needle nosed pliers.  The starboard engine was an exact repeat- the entire thread came out in one piece, with no damage to the heat exchanger.  You may wonder why I didn’t just use a pipe extractor.  First, I didn’t have one in the truck at the time, and second, the plug was so frozen, I doubt it would have worked- the amount of force required would have likely torn the lug off first.  Similarly, you might ask why I didn’t apply heat.  First, the heat exchanger is constructed of silver-soldered brass and bronze, and second, it was filled with seawater, which would have carried the heat away as quickly as it could be applied.

Before I started, I didn’t really know if this technique would work.  I had little to lose, however, as the damage was done, largely before I even arrived.  Anyway, if I failed, any additional damage would be minimal compared to the process of replacing the heat exchanger.  Besides, it was nearing the end of a raw winter’s day in a dead cold engine room, and I just wanted to go home.

Now, of course, I can recommend this method as a way out of a tough situation.  A construction reamer is designed to work on steel/iron, so this technique is not limited to brass fittings.  Note that a reamer with a counter-clockwise-spiral will not grab and ‘self-feed’ into the hole, which allows for greater control.

Not previously mentioned, but the hardest part of this job was getting into position to do the work.  But that’s another story.

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