KTM 125 SX [2021-2022]: A Lightweight Two-Stroke Dynamo Built for the Dirt

Introduction

The KTM 125 SX has long been a staple in the motocross and enduro world, offering riders a gateway into competitive dirt biking without sacrificing the thrill of a high-revving two-stroke engine. The 2021-2022 model generation refines this legacy with modern upgrades like fuel injection and premium suspension components. Designed for riders who live for sharp turns, airborne jumps, and the smell of premix, this bike balances aggression with accessibility. After spending a day wrestling this orange machine through muddy trails and sandy berms, it’s clear why it remains a favorite for both newcomers and seasoned riders looking to sharpen their skills.

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Design & Ergonomics: Purpose-Built for Aggression

The KTM 125 SX’s design screams "race-ready." Its white-and-orange livery (with blue accents on the 2022 model) is unmistakably KTM, while the minimalist bodywork keeps weight down to a feathery 87.5 kg (193 lbs). The bike’s central double-cradle steel frame feels robust yet flexible, absorbing impacts without transferring every rock and root to the rider’s palms.

Standing at the bike, the adjustable seat height (listed as 850 mm/33.5" in 2021 and 950 mm/37.4" in 2022 data) initially seems intimidating, but the narrow profile lets shorter riders plant at least one foot firmly. The handlebars are positioned for an aggressive riding stance—perfect for attacking whoops or leaning into corners.

Fuel capacity is limited to 7.5 liters (1.98 gallons), a reminder this machine is built for sprint laps, not cross-country marathons. Still, the compact tank design ensures the bike stays nimble mid-air.

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Engine & Performance: Two-Stroke Fury, Modernized

At the heart of the 125 SX lies a 124.8cc liquid-cooled two-stroke single-cylinder engine producing 15 HP (10.9 kW). But don’t let the modest numbers fool you—this engine is a riot. Thanks to KTM’s Transfer Port Injection (TPI), the days of premixing fuel are gone. The fuel-injected system delivers precise air-fuel ratios, resulting in crisp throttle response and reduced exhaust smoke.

The powerband hits hard around 8,000 RPM, launching the bike forward with a surge that demands respect. Clutch control is critical here, and the Brembo hydraulic clutch offers buttery modulation, preventing stalls during technical climbs. Paired with a 6-speed gearbox, the engine thrives in varied terrain. First and second gears tackle tight trails, while fourth through sixth let you stretch its legs on open fire roads.

Two-stroke purists might miss the raw "hit" of carbureted rivals, but the TPI system’s reliability and reduced maintenance are wins for weekend warriors.

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Handling & Suspension: WP’s Magic Carpet Ride

Where the 125 SX truly shines is its chassis and suspension setup. The WP XACT-USD 48 mm front forks and WP XACT Monoshock rear suspension offer 310 mm (12.2") and 300 mm (11.8") of travel, respectively. Dialed in for motocross, they absorb big jumps with ease but remain supple enough for trail chatter.

During our test ride, the bike felt planted in rutted corners, with the 26.1° rake angle striking a balance between stability and quick steering. The Brembo single-disc brakes (220 mm front/rear) provide ample stopping power, though aggressive riders might upgrade to sintered pads for harder bites.

The 110-17 (front) and 150-17 (rear) Maxxis tires grip predictably in intermediate terrain, though serious mud riders may opt for knobbier replacements—something MOTOPARTS.store stocks in spades.

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Competition: How It Stacks Up

The 125cc two-stroke class is fiercely competitive. Here’s how the KTM holds its ground:

• Yamaha YZ125: The carbureted YZ125 is lighter (84 kg) and slightly cheaper but lacks the KTM’s fuel injection. Its suspension isn’t as plush, and the 5-speed gearbox feels dated next to KTM’s 6-speed.
• Husqvarna TC125: Sharing KTM’s engine and frame DNA, the Husky differentiates itself with a composite subframe and subtle styling. It’s a coin toss for riders loyal to either brand.
• Beta 125 RR: Beta’s offering leans toward enduro with a wider-ratio gearbox and softer suspension. It’s better for trail riding but less explosive on MX tracks.

The KTM’s blend of cutting-edge tech (TPI, hydraulic clutch) and race-ready components makes it the top pick for track-focused riders.

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Maintenance: Keeping the Orange Beast Alive

Two-strokes are simpler than four-strokes, but the 125 SX still demands attention:

1. Air Filter: Clean after every ride. The bike’s high-revving nature sucks in dust like a vacuum.
2. Spark Plugs: The NGK BR9ECMIX plug is critical for smooth ignition. Carry spares.
3. Chain & Sprockets: The 520 O-ring chain lasts longer with regular lubrication. Check alignment to avoid premature wear.
4. Cooling System: Use high-quality coolant (“WASSER+” per manual) and inspect hoses for leaks.
5. Suspension: Rebuild WP forks/shock annually if ridden hard. MOTOPARTS.store stocks OEM seal kits.
6. Brake Fluid: Flush with DOT 4/5.1 annually to maintain lever feel.

Pro Tip: The Brembo clutch is durable but benefits from periodic bleeding to retain its featherlight pull.

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Conclusion: A Thrilling Entry to Bigger Things

The KTM 125 SX [2021-2022] is a near-perfect training tool for riders honing their skills or veterans craving a lightweight thrill. Its fuel-injected two-stroke engine eliminates carburetor fuss, while WP suspension and Brembo components deliver premium performance out of the crate. Whether you’re blasting through berms or swapping sprockets for tighter trails, this bike rewards aggression with adrenaline.

And when it’s time to replace worn parts or upgrade for sharper performance? MOTOPARTS.store has you covered—from chains to clutch plates, we’ll keep your KTM biting harder than ever.

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Specifications sheet

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