Zach, I noticed in the Comanche Flyer that you do not recommend leaving an engine sump type preheater plugged in for prolonged periods, but you gave no explanation why. A recent post and article seemed to say the oposite. Whats your reasoning? Ray B

Ray,
This is 100% the opinion of all of the major engine shops I deal with. The engine sump heaters warm the oil, and vaporize the suspended water resident in the oil. The top of the engine, ie the cam and exposed cyl walls stay significantly cooler than the sump, and as such are the areas that the vapor condenses on. The vapor and the by products of combustion make a very corrosive mixture. If the engine is run and the aircraft is flown, it is not an issue. If it sits there...bad news. As you know, the engine is not sealed. The breather allows ambient air/humidity into the engine all the time. Contrary to popular belief, this is not an exit only hole. If the engine had a constant uniform temperature, the danger would be much less...but...it is an air cooled engine, with a single point of heat. There are no constant temps anywhere on the engine. This was just a big segment of the common issues being discussed at the IND IA renewal seminar last week.
-Zach

Thanks Zach, Good info to have in ones portfolio. Unplugging now! Ray B

Not exactly what I'd call an unbiased source, but, in general, most of it sounds reasonable and worth reading.
http://www.tanisaircraft.com/docs/Aircr ... ptions.pdf
Note the parenthetical assertion in the paragraph 3rd from the bottom on page 5.

"With a Tanis preheater, the airplane is simply “plugged in” to 110-volt outlet for about 5–6 hours prior to being started. (It can be left plugged in continuously, keeping the engine constantly ready for starting)."

In context, the above comment is referring to the type of heater that heats the cylinders as well as the sump. His comments about heating only the sump include (on page 3 under "Drier Consequences"):

If only the engines lower end (oil sump) is heated, the moisture vapor rises and condenses on the cold parts such as the crankshaft and cam. ... This moisture will produce rust and acids."