From
https://www.lycoming.com/content/frequency-flight-and-its-affect-engine
We have firm evidence that engines not flown frequently may not achieve the standard expected overhaul life. Engines that are flown occasionally deteriorate much more rapidly than those that fly consistently. Pilots have asked, “What really happens to an engine when it’s flown only one or two times per month?” An aircraft engine flown this infrequently usually accumulates rust and corrosion internally. This rust and corrosion are often found when an engine is torn down.
Some operators are running the engines on the ground in an attempt to prevent rust between infrequent flights. This may harm rather than help the engine if the oil temperature is not brought up to approximately 165˚ F because water and acids from combustion will accumulate in the engine oil. The one best way to get oil temperature to 165˚ F is to fly the aircraft. During the flight, the oil normally gets hot enough to vaporize the water and most acids and eliminate them from the oil. If the engine is merely ground run, the water accumulated in the oil will gradually turn to acid, which is also undesirable. Prolonged ground running in an attempt to bring oil temperature up is not recommended because of inadequate cooling that may result in hot spots in the cylinders, baked and deteriorated ignition harness and brittle oil seals which cause oil leaks.
From
https://www.lycoming.com/content/operating-cold-weather
Engine operating temperature is another item that is not usually given enough consideration in cold weather. We usually are very cautious about high oil temperature which we know is detrimental to good engine health, while a low oil temperature is easier to accept. The desired oil temperature range for Lycoming engines is from 165˚ to 220˚ F. If the aircraft has a winterization kit, it should be installed when operating in outside air temperatures (OAT) that are below the 40˚ to 45˚ F range. If no winterization kit is supplied and the engine is not equipped with a thermostatic bypass valve, it may be necessary to improvise a means of blocking off a portion of the airflow to the oil cooler. Keeping the oil temperature above the minimum recommended temperature is a factor in engine longevity. Low operating temperatures do not vaporize the moisture that collects in the oil as the engine breathes damp air for normal combustion.* When minimum recommended oil temperatures are not maintained, oil should be changed more frequently than the normally recommended 50-hour change cycle. This is necessary in order to eliminate the moisture that collects and contaminates the oil.
*This part is misleading. Almost all the water comes from combustion, not "damp air." An airplane on the dry prairies in winter will still get water in the crankcase. Refer back to the first quote where Lycoming says "
because water and acids from combustion will accumulate in the engine oil."
From https://www.lycoming.com/content/low-time-engine-may-not-mean-quality-and-value
The point of this discussion is simple. A low-time engine may not add value to an aircraft, and the buyer should be aware of all factors which may affect the condition and value of the engine. An engine which is not flown frequently is subject to deterioration as a result inactivity. When the engine does not achieve flight operating temperatures on a regular basis, the moisture and acids that form as a result of combustion and condensation are not vaporized and eliminated through the exhaust and crankcase breather. As moisture and acids collect in the engine, they contribute to the formation of rust on the cylinder walls, camshaft and tappets.
As the engine is run after rust has formed, the rust becomes a very fine abrasive causing internal engine wear, particularly to the camshaft and tappets. As these components wear, they make more metal which attacks the softer metals in the engine. Piston pin plugs are examples of parts that may wear rapidly when rust becomes an abrasive inside the engine. This wear could eventually lead to failure.
Ram's video:
If you don't want to watch the whole thing, skip to the 3:10 mark.
Note: that picture above is the result of a saltwater corrosion test, not typical internal corrosion.
