Two-strokes come in several types and designs. Each has its own strong and weak areas. The new pilot should be concerned with a few of these varied designs because they are important for proper engine selection. Look at the photo of the two .60 engines. The engines are called “.60s” because the piston displaces .60 cubic inch as it travels.

The total volume of the space—stroke length multiplied by the cylinder’s cross-sectional area—occupied by the piston in its travels is called the engine’s "displacement." Most sport two-strokes used on trainers have a .25-.61 cu. in. displacement.

The larger the displacement, the more powerful the engine and the larger the propeller it can use. But larger-displacement engines use more fuel per minute and cost more. Larger engines also require larger airframes that could cost more.

Look carefully at the two .60-size engines’ sides in one of the photos. One engine has a straight cylinder and one has a raised area (under the “S”). That raised area is the space for the boost transfer ports designed and patented by Dr. Schnuerle in the late 1930s but not first used until after World War II. “Schnuerle porting” is nothing more than extra transfer ports that permit extra air/fuel mixture to flow into the combustion chamber.

Some engines have several extra boost ports. Almost all sport engines made today have at least one such Schnuerle boost port. Schnuerle engines use more fuel and are slightly harder to adjust at idle but are more powerful than non-Schnuerle engines.

Another point to consider is the crankshaft’s support system. If you look at Diagram 2 of the O.S. .25 FX, you’ll see a front and rear ball bearing supporting both crankshaft ends. Diagram 1 of the .40 LA does not have ball bearings. Instead, there is a bronze bushing in the crankcase housing. The crankshaft slides into, and is supported by, this bushing.

Bronze bushings provide more crankshaft support area but also more crankshaft friction that robs engine power. Ball bearings provide better support because they reduce friction. But ball bearings are also more susceptible to corrosion if not properly maintained. Bronze bushings tend to wear sooner than ball bearings and are more difficult to replace. But bushings do not rust or corrode. Electric starting is less problematic with ball bearings; bushed engines require a lighter “touch” when pressing the starter against the propeller.

There are two main two-stroke design types: ABC (AAC) and ringed piston. Ringed-piston engines have one or more compression rings on the piston, just like an auto engine. The rings provide drag and do not have as tight a seal against the cylinder wall as ABC engines do. The ABC type is unique to model engines. The piston is made from Aluminum and the cylinder is made from Brass plated with Chrome. The piston actually has the same diameter as the top of the cylinder does. This can be felt when turning the propeller. The propeller rotation tends to “stick” as the piston reaches the top of its stroke and enters the section of the cylinder with the same diameter.

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How can this work? The varied construction materials expand at different rates as the engine heats when running. The cylinder expands more than the piston does. The final diameter of both allows some clearance at TDC for the piston to move when the engine runs. This makes for a tight compression seal at the top end without ring friction, which could absorb some power. But many non-long-stroke ABC engines have slightly lower torque than ringed engines since compression drops slightly as the piston progresses downward into the larger cylinder area. This expansion difference also provides protection if the engine overheats.

When overheating, the cylinder continues to expand faster than the piston, providing extra clearance that protects these vital parts. But the engine still sags, reducing power, while the destruction process begins in other parts of the engine.