The KTM 125 SX is what KTM themselves call the first bike to buy to start your racing career. This 2021 model is a pure racing machine, built with a lightweight chassis and frame cradling a 125cc two-stroke single. That little screamer of a thumper pushes out 37.5 HP, which is the most of any 125cc two-strokes on sale today by the European manufacturer.
KTM then gets down to business. A grippy race seat is fitted, which is also quite comfortable for longer distances in cross-country races. A NEKEN racing aluminum handlebar is then fitted, and a lightweight KTM racing exhaust allows for little to no backpressure, letting the engine breathe freely as it needs to.
WP suspension has been fitted to the bike, as with all their competition bikes. The clutch is a multi-plate unit with reliable Brembo hydraulics. The full size wheels are fitted with competition grade Dunlop GeoMax MX 3S tires, everything you’d expect from KTM Motorcycles. A truly unique feature of the 125 SX is that if the footpegs are too low, they are interchangeable with the footpegs from the 350 EXC-F, which sit half an inch higher.
The 2021 KTM 125 SX starts at $7,399 USD / $8,799 CAD.
On this page:we’ve curated specs, features, news, photos/videos, etc. so you can read up on the new 2021 KTM 125 SX in one place.
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.
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.
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.
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.
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.
The frame and subframe on the KTM 125 SX caters for the aggressive riding styles associated with 125 cc racing. The frame itself is professionally crafted from hydroformed chromium molybdenum steel tubes that are robot welded, ensuring the highest levels of consistent quality. The stiffness has been precisely tailored for the most dynamic handling, and works in perfect harmony with the suspension to provide nimble agility without losing high-speed stability. The frame also features “no dirt” footpegs, which have a simple but effective self-cleaning system that ensures the footpegs return to their original position, providing complete control in all conditions. All SX frames feature the same highly durable powder coating as the KTM Factory machines and come with standard frame protectors. The subframe is made from lightweight aluminum, providing rear fender stability and strong support for the rider at little extra weight
The one-piece swingarm is manufactured using a gravity die-cast production process for exceptional strength at the lowest possible weight. It offers a wide range of adjustment for the rear wheel, allowing for a shorter wheelbase on tighter, twisty tracks, or a longer wheelbase for greater stability on longer, more choppy tracks. The swingarm’s chain measurement markings are visible from above, making adjustment simpler and more accurate.
All KTM SX models feature a seat that perfectly matches the range’s dynamic performance-oriented ergonomics. The seat cover features silicone strips that provide better feel and grip for the rider. The seat is conveniently fixed to the bike using one long, lateral screw, making it secure and easy to handle.
All full-size KTM SX models are equipped with the same high-strength aluminum tapered handlebar, made by NEKEN. The handlebar is fixed in rubber damped mountings, which allow for two different positions. The handlebars come standard with an ODI vulcanized grip on the right side and a comfortable ODI lock-on grip, which does not require any wire or glue, on the left. They also feature a bar-pad with a cover that can be attached and removed with little effort.