2004 KTM 125SX

2004 KTM 125 SX: More Power and a Major Suspension Step Forward

The 2004 KTM 125 SX was a significant development of KTM's already formidable small-bore motocrosser. It was not an all-new generation, but the combination of engine, suspension, chassis and ergonomic changes was substantial enough to make the 2004 machine noticeably different from the 2003 model. By this point KTM's 125 had already established a reputation for having one of the strongest engines in the class. The job for 2004 was to make the rest of the motorcycle work as well as the motor.

KTM retained the familiar 124.8 cc engine with its 54.0 x 54.5 mm bore and stroke, five-speed gearbox and hydraulic clutch, but improved the way it breathed. A newly tuned exhaust system was designed to increase midrange and high-rpm performance, while a Moto Tassinari VForce reed valve became standard equipment. Riders had already been fitting VForce systems to earlier KTMs to sharpen throttle response, so KTM effectively incorporated a popular race modification into the production motorcycle. The Keihin PWK 39 mm flat-slide carburettor remained, and owners working on worn or incomplete machines can compare options through the MXOwners carburettor reference.

The result was an engine that preserved the strong midrange character of the previous model while pulling harder toward the top of the rev range. KTM's 125 engines had become one of the company's greatest strengths, particularly in American racing, and the 2004 model benefited directly from lessons learned through competition. Grant Langston's success on KTM machinery had helped prove that the Austrian manufacturer's 125 programme could compete directly with the established Japanese brands.

The larger improvement for many riders was underneath the chassis. The 48 mm WP fork received revised valving aimed at reducing mid-stroke harshness and producing a more progressive action. Front travel remained a generous 300 mm, and the standard specification called for SAE 5 fork oil with a 90 mm air chamber. Owners rebuilding the original WP units can use the MXOwners fork oil reference when comparing service information and settings.

At the rear, KTM made an important change to the WP PDS system by moving away from the progressive-rate spring arrangement used on previous models and adopting a straight-rate spring with revised damping. Earlier PDS-equipped KTMs had attracted criticism for feeling harsh over smaller bumps while still using too much travel on larger impacts. The 2004 revision was designed to let the shock move more freely and make the rear end more predictable. Rear-wheel travel was 335 mm, and careful sag and damping setup remained particularly important on the linkless PDS chassis. The MXOwners rear suspension reference can help owners identify and understand rear suspension components and setup.

KTM also worked on rider ergonomics and chassis behaviour. Taller handlebars corrected the unusually low bar position of the previous model, while detail changes throughout the motocross range were aimed at improving chassis rigidity, balance and control. The central chrome-molybdenum steel frame, aluminium swingarm, 21-inch front wheel and 19-inch rear remained recognisably KTM, as did the strong 260 mm front and 220 mm rear disc brakes.

For owners today, the 2004 KTM 125 SX represents an important stage in KTM's transition from building exceptionally fast but sometimes demanding motocross bikes to producing more complete race packages. The engine was already a class strength; the real progress for 2004 was making the suspension and chassis better able to exploit it. It retained the lightweight, high-revving personality that makes a 125 two-stroke entertaining, while incorporating several changes that racers had previously been making themselves. That combination makes the 2004 125 SX a particularly interesting machine from KTM's rapid rise during the early 2000s.