Rotax 122 vs. Rotax 123: Which Aprilia RS 125 Engine Do You Have?

If you own an Aprilia RS 125, you know that not all engines are created equal. Depending on the year of your bike, you are likely running either the Rotax 123 or the Rotax 122.

Knowing which one is sitting in your frame is the difference between ordering a piston that fits and a piston that becomes a very expensive paperweight. In this guide, we’re breaking down the technical differences so you can maintain your RS 125 with confidence.

The Evolution of the 125cc Powerhouse

The Rotax 123 was the heart of the early RS 125 models (Extrema and early Chesterfield editions) until approximately 1995. It was a rugged, powerful unit but was eventually replaced by the Rotax 122, which powered the iconic "Replica" models and the later "Spanish's No.1" and "Poggiali" editions through to the mid-2000s.

While both engines share the same 56 mm bore and 50.6 mm stroke, the internals and electronics are worlds apart.

Key Technical Differences

Feature	Rotax 123 (Early Models)	Rotax 122 (Later Models)
Ignition System	SEM (Commonly Red/Blue)	Nippon-Denso
Clutch Adjustment	External (on the casing)	Internal (under the clutch cover)
Crankshaft	Heavier, thicker webs	Lighter, improved balance
Starting Mechanism	Kickstart (usually)	Electric Start only (standard)
Gearbox Oil	600cc (75W-90)	600cc (75W-90)
Spark Plug Gap	0.5 mm – 0.6 mm	0.5 mm – 0.6 mm

1. The Ignition Headache: SEM vs. Denso

The easiest way to tell the difference at a glance is the ignition. The Rotax 123 uses an SEM system. These are notorious for failure after decades of heat cycles and are becoming harder to source.
The Rotax 122 moved to a Nippon-Denso system, which is generally considered more reliable and has better parts availability for modern riders.

2. The Power Valve (RAVE) Setup

While both use the RAVE (Rotax Adjustable Variable Exhaust) system, the way they are actuated can differ. On the Rotax 122, the solenoid is mounted more cleanly, and the cable routing is simplified. On both engines, ensuring that the valve blade is cleaned of carbon every 3,000 miles is the "survival" secret to keeping that 7,800 RPM power hit alive.

3. Cylinder Compatibility

Warning: You cannot easily swap cylinders between these two engines. The mounting stud patterns and the coolant port designs are different. If you are buying a performance barrel (like an Italquit or Polini), double-check that it specifies your exact engine code.

Final Verdict: Which is better?

The Rotax 123 is often praised by purists for its "raw" feel and slightly beefier construction. However, the Rotax 122 is the choice for the modern "survivalist" due to the lighter crankshaft (which allows the engine to rev faster) and the abundance of spare parts.

No matter which engine you have, the key is the same: high-quality synthetic oil and regular top-end inspections.