APRILIA RS125 and the ROTAX
Aprilia RS125: Understanding the Rotax 122 and Rotax 123 Engines
The Aprilia RS125 evolved from its predecessor, the Aprilia AF-1, and throughout its production history, the RS125 was powered by two different engines: the Rotax 123 and the Rotax 122.
Despite their similar numbering, the two engines have significant differences in terms of technical design, mechanical components, and electronic systems.
Interestingly, the numbering can be somewhat confusing because the Rotax 122 is actually the newer engine, while the Rotax 123 is the earlier design.
The 1988 Aprilia AF-1 commonly used the Rotax 123, while early-generation RS125 models produced from 1993 to 1997 were also equipped with the Rotax 123. From around 1997 onwards, the RS125 began transitioning to the Rotax 122, which remained in use until production of the 125cc two-stroke RS125 eventually ended.
Different Chassis and Engine Mounting
The change from the Rotax 123 to the Rotax 122 was not simply an engine swap.
The early RS125 models equipped with the Rotax 123 used a chassis configuration that differed from the later models fitted with the Rotax 122. The frame dimensions, engine mounting points, and suspension-related components were also different.
This means the two engines should not be considered direct drop-in replacements without taking the corresponding chassis and mounting differences into account.
Different Starting Systems
The two Rotax engines also use different electric starter systems, including differences in their internal mechanisms.
Another noticeable difference is the RPM reading system. The Rotax 122 uses an electronic RPM signal, while the Rotax 123 uses a mechanical system.
Different Carburettors
The carburettor specifications also distinguish the two engines.
- Rotax 122: Dell’Orto PHBH28
- Rotax 123: Dell’Orto VHSB34
This difference reflects the different engine configurations and fuel-delivery requirements of the two generations.
Different Clutch and Gearbox Components
The clutch system is also different between the Rotax 122 and Rotax 123. Differences can be found in the clutch installation, sprocket gearing, and gearbox shaft configuration.
These mechanical differences are particularly important when restoring an RS125 or attempting to interchange components between the two engine generations.
Cylinder and Engine Block Differences
The cylinder markings provide another easy way to distinguish between the two engines.
The Rotax 123 cylinder carries “Aprilia” markings, while the later Rotax 122 cylinder can be identified by additional “Gilardoni 1” and “Gilardoni 2” markings.
The differences go beyond the markings. The two cylinders feature different casting designs, with variations in the cylinder mounting arrangement and port dimensions.
These changes reflect genuine differences in the engine architecture rather than simply cosmetic updates.
Different Ignition Systems
The ignition systems are another major distinction.
The Rotax 122 uses a Denso CDI system, while the Rotax 123 uses an integrated SEM magneto system.
Although both engines powered the legendary Aprilia 125cc two-stroke platform, the Rotax 122 and Rotax 123 are substantially different powerplants.
For collectors and enthusiasts restoring an Aprilia AF-1 or RS125, understanding these differences is crucial — especially when identifying engines, sourcing replacement parts, or considering an engine conversion.
The numbers may suggest that the Rotax 123 came after the Rotax 122, but in the world of Aprilia two-strokes, it is actually the other way around.