Scrolling through controller.com. I noticed this, "121.3 SCMOH". Does anyone know what this means?


1 Answer 1


The C refers to "Channel Chrome" (SCMOH = Since Chrome Major Overhaul), a plating process used to restore worn-oversize cylinder bores. What they do is electroplate the cylinder bores to bring the bore dimension into spec, then reverse the process a little bit, just enough to get de-plating started.

The de-plating causes chrome start to be removed as a web of fissures, that would eventually spread to the entire surface area if it wasn't stopped as soon as the initial fissures form. You then end up with a surface like the cracked mud of a dry lake bed, at the microscopic level.

The web of microscopic surface fissures act like little ditches where oil is collected and retained for lubrication of the cylinder walls, because the basic chrome surface is too smooth (a normal cylinder has micro-grooves created by the honing process) to retain sufficient oil to lubricate the piston rings.

Chrome has significant plusses and minuses, and always refers to rebuilt cylinders, so if an engine has chrome jugs it's important to advertise it up front right in the acronym. The cylinders have orange paint on the fins around the spark plugs to denote channel chrome.

On the plus side, chrome bores are long lasting and eliminates cylinder wall corrosion if the plane sits a lot. On the downside, they take forever to break in, and the oil burn will be higher than steel, nitrided or ceramic-nickel cylinders (I prefer ceramic-nickel). A lot of people will avoid an engine with chrome cylinders.

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    $\begingroup$ This is an excellent question -- but it should be just that: a separate question. Click the [Ask Question] button in the upper right of this page to ask a question. Note: you might want to broaden the question a little to include tire pressure and any other simple maintenance tasks you can think of. $\endgroup$ Apr 12, 2022 at 20:42
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    $\begingroup$ Way beyond that. The owner can do a lot of stuff, changing oil, and a lot of other light maintenance. There is a list somewhere on the FAA website. In Canada you can register your airplane as "Owner Maintenance" and do everything, including inspections, repairs, overhauls, use uncertified parts, as if it was a homebuilt. You have to X out all the serial #s and the airplane can't leave the country because nobody else recognizes the category, so most owners leave it in normal category. $\endgroup$
    – John K
    Apr 12, 2022 at 21:15

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