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HOS | Hardened Oil Syndrome

| June 22, 2013 0 Comments More

Hardened Oil SyndromeHOS stands for Hardened Oil Syndrome, a condition found in older Marklin Z scale locomotives where the loco does not run due to congealed, or hardened, oil in the gears. We utilize a pair of older YouTube videos in the article to illustrate, the quality of these videos isn’t quite up to new video camera standards but the content is very relevant.

In earlier days at Marklin, an oil was applied at the factory that had a nasty habit of congealing and hardening up over time. If locos with this older oil have not been run in some time, HOS often sets in. One thing not to do is to keep applying power to the loco in hopes of loosening up the old oil, please don’t do this as you’ll likely end up damaging your motor.

Watch how several Marklin locos typically behave with HOS in the video below. Also of note, this video features a Gaugemaster track cleaner. As you can see, no amount of power will free these HOS locos.

Generally, the locomotive itself is still fine, but that congealed oil has to go. How to get rid of the oil can depend on how advanced the HOS problem. You’ll need to get inside your loco and determine just how much of the hardened oil is present.

Here’s a very useful page by David A. Karp that shows you what your locos look like under the hood and more important, how to get in there for maintenance.


The next video that show the disassembly of Marklin Z scale 0-6-0 to give you a sense of the inner workings. Yes, it can look a little scary at first but I’ve found that with just a few good tools, a well-lit workspace and patience… you can do this too! (Magnification optional but recommended!).

I know some folks have had some success with an HOS loco by adding more oil straightaway, without a good cleaning first, and then trying to run their trains and break-up the old oil this way. Like many aspects of model railroading there are lots of opinions out there on how to achieve results but often in different ways.

Personally, I like the complete cleaning method as it’s a good chance for me to learn about my trains. That said, I have lost key pieces (such as individual gears) during both a disassembly and the following assembly so this way is not without some risk.

Another option, once you determine your loco has HOS, is to have it serviced by folks familiar with, and qualified, to work on Z scale locos. Once your loco is fully cleaned and re-oiled with a modern oil (such as Labelle 108), regular running should keep it in great shape for years to come.

Ztrains Recommended Service for Marklin, AZL and MTL:

Don Fedjur
Carlsbad, CA

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Category: Introduction To Z Scale

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