Breaking in The Engine

The process of breaking in the engine is critical to the operation and longevity of your engine. All of the components of the engine are manufactured to tight tolerances at the factory.

As you are well aware, medal expands when heated. This means the shapes of all the parts (pistons, rings, sleeves) begin to change shape as the engine heats up. Since the engine doesn’t heat up uniformly, all of the parts become distorted. The purpose of breaking in the engine is to allow the “distorted” parts that move against one another to be “mated” for the temperature range that the engine will be ran. If the engine is not broken in properly you will not get the maximum power and performance the engine is capable of and the life of your engine will be shortened.

If you ask 100 “engine experts” how to go about breaking in the engine you will get 100 different break-in procedures. Every engine manual includes a detail break-in procedure including what percentage of oil and nitro should be used. I recommend following the break-in procedure outlined in the manual that comes with your engine. It is worth noting that Evolution Engines offers a line of engines that are broken in at the factory.

I am not going to go through a detailed procedure for breaking in the engine that would ultimately put you to sleep. It is important to know that ABC engines have to be broken in differently than ringed engines. Keep in mind that just about all four-strokes are ringed engines.

Breaking in Ringed Engines

With a ringed engine the object is to run the engine at a very rich fuel setting with a reduced load for short periods of time, maybe 30 seconds for the first run. A propeller one inch smaller in diameter than the propeller you plan to fly your airplane with should be used to reduce the load on the engine. The engine needs time to cool to ambient temperature after each run. Each run will be a little longer with the high speed needle valve adjusted a little leaner. After completing the “ground” break-in procedure from the manual, a ringed engine will require approximately 20 flights before the high speed needle valve can be adjusted for optimal power.

Many times the manual will tell you to adjust the high speed need valve until the engine is on the verge of “four stroking”. All this really means is to adjust the high speed needle valve so rich that the engine runs very rough, then lean the high speed needle valve just enough that the engine runs smoothly. Because a 4-stroke engine only fires on every other stroke, “four stroking” is a common term in the hobby that refers to a 2-stroke engine that is running so rich that it is not firing on every rotation. Many times it is necessary to leave the glow igniter clipped to the glow plug in order to keep the engine running in this “four stroking” mode.

Breaking in an ABC Engine

From the previous page you know that an ABC engine is constructed with an (A) aluminum piston that runs in a (B) brass liner that is (C) chrome plated. The brass liner will expand more than the aluminum piston when heated. The top of the sleeve sees much more heat from the combustion process because this is where the "explosions" take place. For this reason the liner and sleeve are sized so that they fit properly when the engine is at operating temperature. When the engine is cold the liner has a much smaller diameter at the top of the stroke. You will notice that the piston seems to “stick” at the top of the stroke when cold. It may even be necessary to heat the engine with a heat gun before starting it for the first time.

When breaking in an ABC engine it is very important to get the engine to operating temperature as soon as possible. This is opposite of the ringed engine break-in procedure. If an ABC engine is ran cooler than the normal operating temperature for any length of time the piston will begin to wear prematurely against the chrome plated brass sleeve. The engine needs to be at the operating temperature so that the sleeve is expanded to give proper clearance with the piston.

If the engine is ran too cool (or too rich), the piston will wear to fit the brass liner at this cool temperature. Then when the engine reaches operating temperature there will be a gap between the sleeve and cylinder. This will cause the engine to sputter and die all of the time when running at the normal operating temperature.

On the same token, it is important not to run the engine too hot (or too lean) while breaking in the engine. Breaking in the engine too hot will cause the sleeve to expand more than the piston creating a gap. If the gap is large enough combustion gases will blow by the piston damaging the engine. If you follow the directions that come with the engine you should have nothing to worry about.

Some people recommend breaking in an ABC engine with only one moderately rich break-in run on the ground before flying the airplane. Some people recommend flying the airplane with the engine straight out of the box. Again, it is important to following the procedure outlined in the user manual that comes with your engine. Either way, it typically takes about 10 flights before you can lean the engine out to optimal power, which is about half the time needed to fully break in a ringed engine.

Tips for Breaking in The Engine

Always make needle valve adjustments from BEHIND the prop. Never try to reach around the propeller!
Make sure all mounting bolts and engine bolts are tight while breaking in the engine.
Before starting the engine for the first time, remove the head and back plate of the engine and flush it out with fresh fuel. You will find that there is a significant amount of shavings and debris left from the machining process.
Never turn an ABC engine over by hand until it is lubricated with fresh fuel. The friction from the tight fit at the top of the stroke may cause damage to the piston or sleeve if not lubricated properly.
It is always a good idea to heat a new ABC engine with a heat gun to open the clearance between the sleeve and piston before starting it for the first time. If it is very difficult or impossible to turn over by hand you will have no choice but to use a heat gun.
Check the condition of the glow plug after the initial break-in. You may find it full of small medal shavings. If that’s the case you should replace the plug to minimize the chances of the plug failing while your airplane is in the air.

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