2006 KTM 125SX

2006 KTM 125 SX: GP Engine Development and the Return of Six Speeds

The 2006 KTM 125 SX was a significant update and one of the more interesting small-bore KTMs of the mid-2000s. KTM had already built a reputation for producing exceptionally strong 125 two-stroke engines, but for 2006 it pushed the production bike closer to its Grand Prix machinery. The changes included a new cylinder and port layout, revised power-valve components, a V-Force 3 reed system, a six-speed gearbox and further chassis and suspension development. Rather than simply refining the previous model, KTM was clearly using lessons from top-level racing to sharpen the production 125 SX.

The 124.8 cc engine retained its 54.0 x 54.5 mm bore and stroke, but the cylinder, transfer and exhaust-port layout were revised. KTM described the new cylinder arrangement as being closely related to its GP engine, while the power-valve flap and exhaust flange were also redesigned. A V-Force 3 reed-valve assembly replaced the previous intake arrangement, improving airflow into the crankcase. Fuel continued to be metered by a Keihin PWK 39 mm flat-slide carburettor. For owners restoring bikes with damaged or heavily worn original carburettors, the MXOwners carburettor replacement guide provides useful alternatives and sizing information.

One of the most noticeable changes was the return to a six-speed transmission. KTM had experimented with a five-speed gearbox on preceding 125 SX models because the engine had enough power to pull wider ratios, but for 2006 sixth gear returned to give racers additional flexibility on faster circuits. Contemporary testers noted that the revised gearbox retained sensible spacing while offering an extra ratio for long straights. The hydraulically operated wet multi-disc clutch remained a KTM trademark, although competition use still demanded regular inspection of plates and transmission oil.

The chassis also continued to evolve. KTM introduced a cast aluminium swingarm designed to improve stiffness while reducing unnecessary weight, while suspension settings were revised at both ends. The 48 mm WP upside-down fork received friction-reducing coatings, new bushes and a more progressive damping setup. Travel remained 300 mm. The WP PDS rear shock retained KTM's linkless design with 335 mm of rear-wheel movement but received revised oil flow and damping intended to increase resistance to bottoming. Owners servicing the original front suspension can compare specifications through the MXOwners fork oil reference.

The 2006 125 SX remained exceptionally light at 92.4 kg without fuel. With a 1471 mm wheelbase, 925 mm seat height and 390 mm ground clearance, it was a compact but serious competition machine. Strong 260 mm front and 220 mm rear disc brakes, a conventional 21-inch front and 19-inch rear wheel combination and KTM's chrome-molybdenum steel frame completed a package designed purely for motocross rather than trail use.

Like all competition 125 two-strokes, the engine depended on correct fuel mixture and regular top-end maintenance. KTM specified premixed fuel rather than oil injection, with 1:40 being the standard mixture specification for the model. The MXOwners premix calculator is useful when preparing accurate quantities for practice or race-day fuel. Transmission capacity was 0.7 litres, with KTM recommending a high-quality 15W-50 oil for the gearbox and wet clutch.

Historically, the 2006 125 SX arrived while four-strokes were rapidly becoming dominant in motocross, yet KTM continued putting serious engineering effort into its two-stroke range. That persistence became increasingly important as other manufacturers reduced their two-stroke development. The 2006 bike showed that KTM was not merely keeping the 125 alive: it was still transferring GP engine ideas, improving suspension and refining the chassis. For owners today, it represents a particularly appealing stage in KTM's evolution, combining traditional lightweight two-stroke simplicity with technology derived directly from the company's increasingly successful racing programme.