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125 SX - 2019


Model Description

The powerful KTM 125 SX has been the market leader in this segment for many years now. And there is good reason for this – with a completely new chassis and power delivered by a lightweight high-performance 2-stroke engine with a power valve and 6-speed transmission, the KTM 125 SX is the first choice for young champions and in model year 2019 will again define the benchmark in the 2-stroke segment to 125cc.
The bike's tried and tested crankshaft has a stroke of 54.5 mm featuring an excellent balancing which reduces vibrations to a minimum. It is possible to increase the displacement of the latest engine generation to 144cc by a replacing the cylinder, piston and cylinder head without changing the crankshaft.
At the core of the engine is a compact cylinder with a 54 mm bore. Compared to the predecessor models they feature a reworked exhaust port which is now CNC milled for a more precise timing and improved interaction with the main power valve, which is made of a new raw part and machined too. Together with a twin-component cylinder head, this has resulted in outstanding engine performance and throughout the entire rpm range.
A newly designed header and a new silencer are key features of the KTM 125 SX exhaust system for model year 2019. The header using the latest 3D stamping technology has been given an optimized shape and, in conjunction with the upgraded silencer internals, results in a fat torque curve and increased power delivery. A welded aluminum bracket replaces the previous plastic collars used to secure the silencer and also helps to reduce weight.

The newly developed DS (diaphragm steel) clutch features a one-piece outer clutch hub combined with a primary gear made of high strength billet steel. This guarantees unbeatable reliability and, due to its compact, rivetless construction, reduces the engine width by 10 mm. Incorporating thin steel carrier friction discs, the extremely heat-resistant clutch plates ensure reliable disengagement of the clutch. An additional benefit is that they are pre-tensioned by means of a specially designed diaphragm spring, which significantly reduces the force required to pull the clutch lever. The Brembo hydraulic system also ensures an easy and precise action.

The new and very robust 6-speed transmission unit in the KTM 125 SX has been reinforced for model year 2019 and comes from a new transmission plant operated by Pankl in Austria which represents the best there is in terms of quality and reliability. KTM has retained the gear ratios from the predecessor and matched them perfectly to the engine's performance characteristics. All gear pairs have now been further optimized for smooth gear changes. KTM's advanced No-Dirt technology is used for the gear shift lever and is designed to prevent mud and dirt from fouling the lever joint and thus guarantees reliable gear changing at all times.

It is common knowledge that the KTM 125 SX engine is the most powerful and most competitive engine in its class. KTM is absolutely convinced that 2-stroke technology, with its outstanding power-to-weight ratio, low costs and uncomplicated engineering, will continue to lead the chase in the future. The 125cc engine was developed using the most advanced engineering expertise KTM has to offer and is the most powerful in this segment over the entire rpm range. Weighing in at a mere 17.2 kg, it is the lightest engine in its class. This gives the rider precisely what he wants – faster lap times as a result of the bike's handling and outstanding overall performance.

The crankshaft of the KTM 125 SX is positioned high up in the die-cast crankcase at a point that is now closer to the center of gravity. This contributes to improved centralization of the masses and at the same time optimizes rideability. The engine unit is extremely compact and is a major factor in keeping down the overall weight of the bike. For model year 2019, KTM has reinforced the seat of the kick-starter intermediate gear for improved reliability. With a new clutch, the new clutch cover is now lower, which has reduced the engine width by 10 mm.

The KTM 125 SX is fitted with a 38 mm flat slide MIKUNI carburetor, which provides a smooth and controllable power delivery over the entire rpm range and offers tremendously precise throttle response.

Model Specification


Engine Type

Single cylinder, 2-stroke


124.8 cc




6 gears

Engine Lubrication

Mixture oil lubrication 1:40


Liquid cooled


Wet multi-disc clutch, Brembo hydraulics





Central double-cradle type


5/8 x 1/4

Steering Head Angle

63.9 degrees

Tank Capacity

7.5 litres


Primary Gear Ratio


Secondary Gear Ratio


Brake System Front

Disc brake 260 mm

Brake System Rear

Disc brake 220 mm


Front Suspension

WP USD, AER airfork 48 mm

Rear Suspension

WP shock absorber with linkage

Suspension Travel Front

310 mm

Suspension Travel Rear

300 mm



54 mm


54.5 mm


1485 mm

Ground Clearance (unloaded)

375 mm

Seat Height

950 mm

Weight (No Fuel)

87.5 kg