Maintaining a Model Aircraft Engine

Other than wiping oil and gunk off the engine and making sure to put oil in it to prevent rust, there is not a lot of maintenance required.  In fact, it is almost never necessary to open an engine at all unless you crash in which case you can clean dirt off the outside of the engine and then remove the carburetor for cleaning.

There is a very small chance that anything actually got inside the engine unless you crashed hard enough to break it or really buried the engine in the dirt.

Also see

• How to Disassemble a Model Aircraft Engine

After-Run Oil

A lot of companies sell so-called after run oil.  None of these companies actually make oil, so they are simply repacking an already available product at a greater cost to the consumer.  I like Marvel Mystery Oil, but it has not prevented the bearings in my engines from rusting.  I don't know why it doesn't work... maybe that's the mystery.  But it has a nice minty scent which is important I guess.

Marvel Mystery Oil works fine as a short term protective agent in your engine. But before you put protective oil in the engine, you have to get all the fuel out.  At the end of the day, some people drain their fuel tank and then run their engine at full throttle until it quits.

This practice does not burn all the fuel out of the engine.  Why?  Because it takes a lot more fuel to run an engine at full throttle than to run at idle which means there may still be fuel in the engine, but not enough for the engine to run with the throttle wide open.

To really get the fuel out of your engine, run the engine at full throttle until it quits.  Then reduce the throttle to idle and repeatedly start your engine and let it run until it quits.  Keep starting the engine until it will not kick over any more.  Now disconnect the glow igniter.

Pour a liberal amount of oil in the carb (full throttle setting) and turn over the engine several times.  Put more oil in and turn the engine over some more.  Do it again.

The after run oil manufacturers say, "Just use a couple drops."  If the engine were open and you could actually see the oil going into the bearings, do you think a couple drops would be enough?  When you put the oil in the carb, you can't be sure it's going where you want it to so use plenty.

Oil is cheap compared to bearings.  Wrap your engine in clean rag held in place with a rubber band so the oil doesn't drip all over everything.  The engine will be safe for a few weeks.

For the most part I don't use after run oil because I use fuel containing castor oil.  I run the fuel out of the engine at the end of the flying day and clean the outside of the engine when I am cleaning the plane.  So far I have not had a problem with rusted bearings with the exception of some engines that were stored for an extended period and supposedly protected by filling the crankcases with Marvel Mystery Oil.

Long-Term Storage

This is how I found out that Marvel Mystery Oil doesn't provide long-term protection.  Several of my engines were stored for a extended period (four years).  When I got them back the bearings were rusted and needed to be replaced.  I had literally filled the crankcase with oil and then put the engine in a freezer bag with the air sucked out.  I thought that would be good.  Unfortunately I was wrong.  There was still oil in the engine but somehow the bearings had rusted.

The next time I stored my engines, I took off the carburetors and oiled them with sewing machine oil.  I then put the carbs in small baggies.  The carb openings were plugged with pieces of paper towel.  I pulled the back plates off the engines and filled the crankcases with transmission fluid and put the engines in freezer bags along with the carb.  These engines came back with no rust.

Of course you will have to remove the back plate from the engine and drain the oil out, but that's not a big deal.  Normally I don't bother trying to remove all the oil.  I just put a good dose of sewing machine oil or Marvel Mystery Oil in the engine after letting the transmission oil drain out for a while.

Cleaning

If your engine has burnt oil all over it then trying to clean your engine by hand can be unsatisfactory.  Clarence Lee suggests using automatic dishwashing detergent in very hot water to clean deposits from engines.

I have tried the method and it is truly amazing.  I boiled some water in a big cast aluminum pot and then dropped in some detergent.  My test engine, a Fox Eagle III .61, came clean but turned a funny color.

Initially I suspected that the discoloring of the engine had to do with the aluminum pot, so I cleaned the next one in a steel pot.  Lo and behold it came clean with no discoloration.

Unfortunately, the next engine I cleaned also discolored so I don't know what the problem is.  If you find the discoloring objectionable then you are taking a risk using this cleaning method.

You only need about 2-3 tablespoons of detergent.  Mr. Lee also said the lemon scented variety will discolor engines.  Because he knows what he's talking about I didn't try it.

Normally, I disassemble the engine and use a clean rag and lots of oil to clean the engine.  I use this method only to clean very dirty engines.  In fact, I almost never take an engine apart to clean it — it's almost never necessary.

Warning! If you use this method to clean your engines, all the oil will be removed.  In other words, anything that can rust will.

Let the parts cool down to room temperature after removing them from the pot.  Then rinse them under cool water to remove the detergent.  Dry the parts thoroughly and then oil them.

	Note: The engine should be disassembled before using this method. This is my O.S. Max 1.08 after being in storage almost 10 years.  It was covered inside and out with brown gunk.  In this photo I have already attempted to clean it with liquid detergent and a toothbrush.  Obviously it still needs work.
	Boil a pot of water and put in about two tablespoons of automatic dishwasher detergent.  I don't measure out the detergent — this is just to show about how much to use. Warning! If you dump the detergent in it too fast when the water is boiling it will boil over.  Add the detergent slowly.
	The engine was removed after about 30 seconds. This is what the water looked like.  If you plan to clean multiple engines you will have to change the water frequently.  Otherwise you can actually stain an engine with dirty water (yes, it happened to me).
	And here is the same O.S. 1.08 looking happy.

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Copyright © 2003 Paul K. Johnson