01. Engine & Exhaust

Engine
The KTM 125 SX engine is undoubtedly once again the most competitive and powerful engine in its class. With an outstanding power-to-weight ratio, low cost and overall simplicity, it is clear that 2-strokes will continue to have a successful future at KTM. The compact and durable 37.9 lbs engine was developed using the latest KTM engineering know-how, resulting in broad power across the rev range. Its easy rideability and outstanding performance are benefits that equate to faster lap times and better results.


Cylinder
At the heart of the 125 cc engine is a reworked cylinder with a 54 mm bore. It features an improved power valve unit with a sophisticated mechanism designed for the lateral support exhaust ports. The new exhaust port layout increases engine performance, while the upper contour of the exhaust port has been machined to ensure more accurate port timing. Together with the two-component cylinder head, the newly reworked cylinder is key to the KTM 125 SX's outstanding performance across the entire rev range.


Crankcases and engine covers
The lightweight engine cases are manufactured using a high pressure die-cast production process, keeping wall thickness to a minimum while retaining strength and durability. The design allows for a high crankshaft position which is located near to the center of gravity, resulting in optimal mass centralization and easier rideability. The engine body is extremely compact, which contributes significantly to the low engine weight. Additionally for 2020, the kickstart intermediate gear has been reworked for improved reliability.


Transmission
The KTM 125 SX is fitted with a robust 6-speed transmission. The gear ratios, which featured improvements and reinforcements, were carried over from the predecessor. The gearbox features a new drive shaft which uses a screw for front sprocket fixation and diaphragm spring instead of a circlip, ensuring secure pinion fixation. Another highlight of the 125 SX transmission is the "no-dirt" gear shifter. Similar to the footpeg design, it prevents the build-up of dirt and ensures the next gear is always available, no matter what the conditions.


Carburetor
The KTM 125 SX is fitted with a MIKUNI TMX 38 mm flat slide carburetor featuring settings that were adapted to suit the new airbox developed in 2019. For 2020, reworked reeds for intake reed block provide further improvements in function, sealing and reliability, ensuring the whole system is always providing smooth, controllable power throughout the rev range.


Ignition
All 2-stroke models feature a CDI control unit with advanced hardware that improves the starting behavior of the engine. The ignition curves remain unchanged.


Cooling
Maintaining the optimal engine temperature is crucial to ensuring consistent horsepower throughout the ride. The KTM 125 SX features newly designed radiators which are mounted 12 mm lower than the predecessors. This lowers the center of gravity and matches the design of the new shrouds perfectly in order to achieve perfect ergonomics. By using CFD technology (computational fluid dynamics) together with a clever coolant circuit routing, the innovative system ensures the optimum engine temperature for the highest performance, no matter what the conditions. The radiators also feature special protectors that serve not only as protection from flying rocks and debris but also as a brace that disperses energy around the radiators in the event of an impact.


Exhaust
When it comes to power delivery and performance, the exhaust developed specifically for this bike plays a key role and packs a punch that is hard to beat. In addition, its special shape provides slimmer ergonomics while still operating safely within the noise limits for racing.
