KTM 125 SX (2016-2020): A Lightweight Two-Stroke Beast Built for Motocross Dominance

Introduction

The KTM 125 SX (2016-2020) is a motorcycle that needs no introduction in the motocross world. As part of KTM’s relentless pursuit of off-road excellence, this generation solidified its reputation as a featherweight champion in the 125cc class. Designed for riders who crave the raw, unfiltered thrill of two-stroke power paired with race-ready agility, this bike is a masterclass in balancing brute force with precision engineering. Having spent time throwing dirt with this machine, it’s clear why it remains a favorite for both amateur racers and seasoned MX enthusiasts.

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Engine Performance: Two-Stroke Fury Meets Refinement

At the heart of the KTM 125 SX lies a 124.8cc liquid-cooled single-cylinder two-stroke engine that’s as explosive as it is iconic. With a bore and stroke of 54.0 x 54.5 mm (2.1 x 2.1 inches), this engine delivers a punchy 35 HP (26 kW)—a staggering figure for its size. The Mikuni TMX 38mm carburetor (featured on most model years) ensures crisp throttle response, while the Kokusan electronic ignition system keeps timing sharp even under brutal RPM loads.

Key Highlights:
- Power Delivery: The two-stroke character is alive and well here. From mid-range onward, the engine screams with a frenetic energy that’s addictive. There’s no “gentle” powerband—just a surge of acceleration that demands respect (and precise clutch control).
- 6-Speed Gearbox: The transmission is slick and well-spaced, letting you exploit every ounce of power. Shifts click into place with minimal effort, even when bouncing through whoops.
- Weight Advantage: At just 87.2 kg (192.2 lbs) dry, the bike feels like an extension of your body. The power-to-weight ratio is ludicrous, making it easy to loft the front wheel or correct mid-air trajectories.

The kickstart ritual is straightforward, though it rewards a firm, committed stroke. Once fired up, the exhaust note—a high-pitched, metallic wail—signals readiness for battle.

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Handling: Agile, Precise, and Unapologetically Aggressive

KTM’s chromoly steel central-tube frame is the backbone of this bike’s razor-sharp handling. With a wheelbase of 1,485 mm (58.5 inches) and a rake angle of 26.5°, the 125 SX carves through corners like a scalpel. The lightweight design (coupled with a 37.8-inch/960 mm seat height) gives taller riders room to maneuver, while shorter pilots can still manage thanks to the narrow midsection.

On the Track:
- Cornering: The bike flicks side-to-side with minimal input. Weight distribution feels neutral, allowing you to pivot on the pegs and attack berms with confidence.
- Jumping: Landing from jumps is where the suspension truly shines (more on that later). The bike stays composed even after awkward takeoffs, and the 14.8-inch (375 mm) ground clearance shrugs off rocky terrain.
- Stability: At speed, the chassis remains planted. You won’t fight headshake unless you’re brutally careless with throttle inputs.

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Suspension and Brakes: WP’s Race-Bred Setup

KTM’s partnership with WP Suspension pays dividends here. The WP AER 48 USD forks (48 mm diameter) and WP shock absorber with linkage offer 300 mm (11.8 inches) of travel at both ends. Preload and damping adjustments are straightforward, making it easy to tailor the setup for different tracks or rider weights.

Suspension Feel:
- Small Bumps: The forks absorb chatter effortlessly, keeping the front wheel glued to the ground.
- Big Hits: Bottoming resistance is impressive. Case in point: overshooting a tabletop resulted in a firm but controlled compression, not a spine-jarring crash.

Braking Performance:
- Front/Rear Discs: The Brembo calipers (mounted on floating bearings) provide strong, progressive stopping power. Lever feel is firm, and the 220 mm (8.7-inch) discs handle repeated abuse without fading.

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Competition: How Does the KTM 125 SX Stack Up?

The 125cc motocross segment is fiercely competitive. Here’s how the KTM holds its ground:

1. Yamaha YZ125:
2. The YZ125 is a legend, but its engine lacks the KTM’s top-end rush. Yamaha’s carbureted setup feels smoother but less explosive.

3. Suspension is plusher for trail riding, but the YZ’s steel frame is heavier (94 kg dry) and less nimble mid-corner.

4. Husqvarna TC125:

5. As KTM’s sibling brand, the TC125 shares much of its DNA. However, Husky’s composite subframe and Magura brakes offer a slightly different feel—softer initial bite but more modularity.

6. The TC125 costs more, making the KTM a better value for budget-conscious racers.

7. TM MX125:

8. The Italian TM packs a ferocious engine and premium components, but parts availability and dealer support lag behind KTM’s global network.

Verdict: The KTM 125 SX strikes the best balance between aggression, adjustability, and accessibility. It’s the bike to beat for riders prioritizing outright speed and modern engineering.

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Maintenance: Keeping Your 125 SX in Peak Condition

Owning a high-strung two-stroke requires diligence. Here’s how to keep your KTM 125 SX running strong—and where MOTOPARTS.store can help:

1. Engine Rebuilds:

2. Two-strokes eat pistons for breakfast. Plan on replacing the piston and rings every 30-40 hours of hard riding. Our store stocks pro-grade piston kits matched to KTM’s bore specs.

3. Carburetor Tuning:

4. The Mikuni TMX carb is reliable but sensitive to altitude and temperature changes. Keep a selection of jets on hand (available in our carburetion section) for quick adjustments.

5. Suspension Servicing:

6. WP forks need fresh oil every 50 hours. Use 5W suspension fluid (metric) or 0.25-0.30 kg/mm springs for heavier riders.

7. Chain and Sprockets:

8. The 520 chain wears quickly under MX abuse. Upgrade to a DID ERT3 gold chain and steel sprockets (13:50 gearing) for longevity.

9. Cooling System:

10. Flush coolant annually with a ethylene glycol mix. Check for leaks around the cylinder head after overheating incidents.

11. Air Filter Care:

12. Clean the foam filter after every ride. Our twin-air filter kits include sealing grease to prevent dirt ingestion.

Pro Tip: Always use DOT 4/5.1 brake fluid in the Brembo system—it’s hygroscopic and degrades faster than you’d think.

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Conclusion: A Modern Classic for Purists and Thrill-Seekers

The KTM 125 SX (2016-2020) is a reminder of why two-strokes still matter. It’s loud, demanding, and utterly thrilling—a bike that rewards skill and punishes complacency. From its race-proven suspension to its explosive engine, every component feels purpose-built for motocross glory.

Whether you’re chasing podium finishes or just love the smell of premix at sunrise, this generation of 125 SX deserves a spot in your garage. And when it’s time to rebuild, upgrade, or tweak, MOTOPARTS.store has the components to keep you ahead of the pack.

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Specifications sheet

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