2007 KTM 125SX

2007 KTM 125 SX: A Complete Redesign for KTM's Two-Stroke Racer

The 2007 KTM 125 SX was a major redesign and one of the most important updates to KTM's small-bore motocross bike of the decade. While many manufacturers were reducing their investment in two-strokes as four-strokes took over modern motocross, KTM went in the opposite direction. The company introduced an extensively redesigned 125 SX with a new frame, new bodywork, a lighter engine top end, revised suspension, a new swingarm and updated electronics. It was clear evidence that KTM still considered the 125 two-stroke an important part of its racing future.

The engine retained the familiar 124.8 cc capacity and 54.0 x 54.5 mm bore and stroke, but the cylinder and cylinder head were completely redesigned. KTM made the top end considerably smaller and lighter, with contemporary reports noting that the new cylinder and head package was remarkably compact. Revised porting increased performance, particularly through the midrange and top end, while a new water-pump impeller improved coolant circulation. Fuel continued to be delivered by a 39 mm Keihin PWK-S flat-slide carburettor, and owners maintaining worn original equipment can compare alternatives through the MXOwners carburettor replacement reference.

The ignition system also moved forward. The Kokusan digital CDI incorporated two selectable ignition curves, allowing the rider to choose between a more aggressive performance setting and a softer delivery by changing a connector beneath the fuel tank. This gave racers a simple way of altering the character of the engine for different track conditions without making mechanical changes. The six-speed gearbox remained, matched to KTM's hydraulically operated wet multi-disc clutch and a standard 13:50 final-drive ratio.

The chassis represented an equally important step. KTM introduced an all-new chrome-molybdenum steel frame and a redesigned cast-aluminium swingarm. The objective was to reduce weight while improving rigidity, rider control and handling consistency. The result was a remarkably light production motocrosser at approximately 89.5 kg without fuel, more than three kilograms lighter than the previous year's machine. The redesigned bodywork also gave the rider more area to grip with the knees and made movement across the seat and tank more natural.

Suspension development was another major part of the 2007 package. The 125 SX received WP's 48 mm closed-cartridge 4860 MXMA CC fork, replacing the previous open-cartridge design. The closed-cartridge system separated the damping oil more effectively and was intended to provide more consistent damping during hard motocross use. Front-wheel travel remained 300 mm. The WP PDS rear shock was also redesigned and worked with the revised swingarm to provide 335 mm of rear-wheel travel. Owners rebuilding the WP front end can use the MXOwners fork oil reference, while rear suspension components and setup information can be compared through the MXOwners rear suspension reference.

The rest of the specification remained focused purely on competition. A 260 mm front disc and 220 mm rear disc handled braking, while the 21-inch front and 19-inch rear wheels carried Bridgestone motocross tyres. The 7.5-litre tank kept the bike slim, and the 925 mm unloaded seat height reflected its uncompromised racing purpose. KTM specified a 1:40 premix ratio using high-quality two-stroke oil, and owners preparing race fuel can use the MXOwners premix calculator to calculate accurate quantities.

Historically, the 2007 125 SX is significant because KTM was investing heavily in two-stroke development at exactly the time much of the industry was moving away from it. Rather than allowing the 125 to become an ageing entry-level model, KTM gave it a new chassis, improved suspension and substantial engine development. That decision helped establish the path KTM would follow for the next two decades, continually developing competitive two-strokes while several major rivals allowed theirs to disappear. For owners today, the 2007 125 SX marks the beginning of a distinctly more modern KTM era and remains an important machine in the history of the company's two-stroke motocross programme.