KTM 125 SX (2007-2010): A Lightweight Two-Stroke Legend Revisited

Introduction

The KTM 125 SX (2007-2010) isn’t just a dirt bike—it’s a rite of passage for riders craving the raw, unfiltered thrill of two-stroke performance. Designed for motocross and enduro enthusiasts, this Austrian-engineered machine balances aggression with precision, offering a ride that feels like strapping a rocket to your back while still demanding technical finesse. Having spent time aboard this orange beast, I can confirm it remains one of the most exhilarating lightweight dirt bikes ever built. Let’s dissect why this generation still commands respect in the off-road world.

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Design & Chassis: Built to Dominate

Frame & Weight Distribution

At the heart of the 125 SX lies its central double-cradle Chromoly steel frame, a design that prioritizes rigidity without adding unnecessary bulk. The dry weight of 90.8 kg (200.2 lbs) feels almost deceptive—this bike is featherlight when wrestling through ruts or pivoting mid-air. The slim profile and 985 mm (38.8-inch) seat height (2009-2010 models) cater to taller riders, though the 2007 model’s lower 925 mm (36.4-inch) seat offers accessibility for shorter inseams.

WP Suspension: Precision Meets Plushness

KTM’s partnership with WP shines here. The 43 mm WP USD forks and WP PDS rear shock (Progressive Damping System) deliver 300 mm (11.8 inches) of front travel and 335 mm (13.2 inches) at the rear. On rough terrain, the suspension soaks up impacts like a sponge, but it’s the mid-stroke support that impresses—perfect for hard landings or aggressive cornering. Later models (2008-2010) refined the damping, reducing arm pump on marathon rides.

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Engine Performance: Two-Stroke Glory Unleashed

The 124.8cc Powerplant

The liquid-cooled, single-cylinder two-stroke engine is a masterpiece of simplicity. With a 54.0 x 54.5 mm bore/stroke and a Keihin PWK 39 carburetor, it’s a snarling beast that rewards aggressive throttle hands. The powerband hits like a sledgehammer around 8,000 RPM, propelling the bike forward with a visceral scream. Yet, the 6-speed gearbox lets you tame (or unleash) the chaos—shift early for technical trails or pin it wide-open on straights.

Lubrication & Cooling

The 1:40 oil-fuel mixture system keeps things simple but demands discipline—pre-mix wrong, and you’ll kiss the top-end goodbye. Liquid cooling ensures consistency even during prolonged abuse, though the radiator’s placement demands caution in rocky terrain.

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Handling & Ergonomics: Dance Partner or Drill Sergeant?

On the Track

The KTM 125 SX thrives on motocross tracks. The 27° rake angle and 1,471 mm (57.9-inch) wheelbase (2008-2010) strike a balance between stability and flickability. It carves berms with surgical precision, and the 220 mm (8.7-inch) front/rear disc brakes offer stoppie-ready bite. The 2007 model’s shorter 1,417 mm (55.8-inch) wheelbase feels even nimbler but trades off straight-line calmness.

Off-Road Credentials

With 390 mm (15.4 inches) of ground clearance, it scoffs at logs and rocks. The 80/100-21 front and 100/90-19 rear tires (Dunlop Geomax in later years) grip like claws in mud but struggle slightly on hardpack. For enduro riders, adding a skid plate is non-negotiable—the aluminum exhaust hangs low and vulnerable.

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Competition: How Does It Stack Up?

Yamaha YZ125 (2005-2010)

The YZ125’s reputation for reliability is legendary, but its 5-speed gearbox feels archaic next to the KTM’s 6-speed. Yamaha’s engine delivers smoother low-end torque, but the KTM dominates in peak power and suspension adjustability.

Honda CR125R (2007)

Honda’s flagship two-stroke suffers from a muted power delivery and heavier chassis. While the CR125R’s build quality is bulletproof, it lacks the KTM’s razor-sharp reflexes and WP suspension plushness.

Husqvarna WR125 (2007-2010)

Husky’s WR125 is a closer rival, with similar weight and a 6-speed gearbox. However, its Marzocchi suspension can’t match WP’s track-tuned damping, and the engine requires more tuning to reach the KTM’s aggression.

Verdict: The KTM 125 SX wins for riders prioritizing power-to-weight ratio and cutting-edge components. However, Yamaha and Husqvarna appeal to those valuing low-maintenance ownership.

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Maintenance: Keeping the Beast Alive

Top-End Rebuilds

Two-strokes live hard, and the 125 SX is no exception. Plan for piston replacements every 30-40 hours of hard riding. Upgrade to a forged piston (available at MOTOPARTS.store) for extended durability.

Carburetor Tuning

The Keihin PWK 39 is finicky in extreme temperatures. Keep a jetting kit handy, and consider swapping to a JD Jetting kit for crisper throttle response.

Suspension Care

WP components demand fresh oil every 50 hours. Use MOTOPARTS.store’s WP Suspension Fluid Kit to maintain buttery-smooth travel. Check fork seals frequently—muddy rides accelerate wear.

Clutch Upgrades

The hydraulic clutch is robust but prone to fade during endurance races. Install a Brembo clutch master cylinder kit (compatible with 2007-2010 models) for consistent lever feel.

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Conclusion: Why the 125 SX Still Matters

The KTM 125 SX (2007-2010) is a time capsule of two-stroke brilliance—a bike that rewards skill and punishes complacency. Whether you’re blasting through whoops or dialing in suspension settings, it’s a machine that grows with you. And when it’s time to rebuild, modify, or upgrade, MOTOPARTS.store has the parts to keep your orange rocket airborne.

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Specifications sheet

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