What is Engine Balancing?

After certifying hundreds of flat, opposed cylinder aircraft engines, and after building more than three hundred thousand engines for general aviation, Lycoming engineers have learned what it takes to produce properly balanced engines. Many engines have been tested over the years and a great deal of data is available to support the building of engines to factory specifications. To put it simply, the subject of engine balance is well understood by Lycoming and is a major consideration in the design and manufacturer of all Lycoming Certified engines.

These statements are contrary to what has been implied by several advertisements and magazine articles. Some reports in several industry publications say your Lycoming-Certified engine would be much better if you would just take the time and spend the money to have it “custom balanced”. The implication is that the manufacturer knows nothing about the importance of balancing. This is absolutely false. Lycoming-Certified engines are very carefully balanced to the degree that is necessary. They are not balanced to a point of absolute perfection because they run at relatively low speeds (compared to some automobile engines) and therefore do not require the degree of balancing being advocated by shops that deal in this specialty.

To justify this excessive emphasis on balancing, the automobile racing engine is often cited as another place where balancing is important. This is like comparing apples to oranges; the two types of engines are not the same. A typical Lycoming direct drive engine is red-lined at 2700 or 2800 RPM while an auto racing engine may operate at more than three times this speed.

In order to delve into the subject a little deeper, several questions were directed to the Senior Analytical Engineer at Lycoming. His responses may help those who have doubts about the quality of engine balancing done at the factory.

Several Lycoming engine components are dynamically balanced. For non-engineers, that means that the proper balance is determined while the part is in motion; it is spinning as it would be during operation. These parts are the starter ring gear supports and the crankshaft. In addition, dynamic counterweights are statically balanced to control the location of the very critical center of gravity.

Balance also includes the control of weight for a number of moving engine components. Items such as piston pins, piston plugs, and piston rings are 100% machined to close tolerances that provide consistent weights. These are three types of piston plug, and although they are interchangeable, each type must be used as a set because of their weight differences. The weight of dynamic counterweights is also carefully controlled.

The second aspect of engine balancing is accomplished by matching some engine parts by weight. Pistons and connecting rods are in this category. Both of these parts are organized in matched sets, by weight, before installation in a Lycoming engine. Should it be necessary to replace one of these matched parts during the service life of the engine, there is a system to keep the balance within specified tolerances. (See “Notes on Replacing Rods or Pistons” in the Maintenance Section of this book.)

Perhaps the engineering answer to other questions may be enlightening – or to the non-engineers, confusing. The point is, Lycoming engineers are well aware of what is needed to make a Lycoming-Certified engine safe and reliable for the long TBO times that are recommended for these engines. Thousands of hours of engine testing followed by days and weeks of data analysis provide the basis for the Lycoming engine design.

The engineering answer as to why there is a need for matching and balancing of engine parts is this: “As a matter of sound design practice, matching and balancing components will load crankshaft bearings in a predictable manner and reduce the reaction loads at the engine mounts.” Just as we suspected – right!

Since those engineers did such a good job on that last question, another was ventured, “Is there any danger or problem with additional balancing by non-factory activities?” Those of us who are pilots will understand some of the response, but it will take those who speak the language of engineering to comprehend the rest. Here it is: “There are occasions when dynamic balance of the prop/engine combination can provide reduced first order vibration, but additional internal balancing of Lycoming production engines is not required nor recommended. The rotating and reciprocating masses of the six and eight cylinder opposed engines are inherently balanced. The rotating masses of the four cylinder opposed design are balanced. The rotating masses of the four cylinder opposed design are balanced. The reciprocating masses of the four-cylinder engine are not balanced as a vibratory inertia moment at second order exists in the plane of cylinder center lines. Matching the weights of components closer will not reduce the second order moment. A redesign incorporating counter rotating layshafts rotating at twice engine speed could be implemented.

“Additional internal balancing contributes little to engine smoothness, and it may even be harmful when the material is removed from highly stressed parts of the engine.”

To summarize, these are the points we have attempted to communicate by providing information about the balancing of Lycoming-Certified engines. First, Lycoming engineers have acquired vast amounts of data through years of engine testing. This knowledge is used to ensure that Lycoming Certified engines are carefully balanced during manufacture. It is also used to formulate a system which allows satisfactory engine balance to be maintained when weight matched parts are replaced during the operational life of the engine. The engine balancing done by Lycoming is part of an organized system which is intended to provide a high-quality product.

The second point is that additional internal custom balancing, by removing material from highly stressed parts is not recommended by Lycoming. Lycoming does currently offer additional custom balancing for Non-Certified/Experimental Engines, sold through Thunderbolt Engines (Factory Custom-Build Shop), but this is accomplished via another method. Due to the sheer number of parts available at the factory, Lycoming is able to satisfy customers’ close to exact balancing demands by weighing and matching parts.