Wossner 8302DB Piston Kit - 2005-2021 Yamaha YZ125 YZ125X Standard Size B

Wossner 8302DB Standard Series Forged 2-Stroke Piston Kit - Standard Size B

Bring the power and precision back to your Yamaha small-bore engine with the Wossner 8302DB Piston Kit. Specifically engineered for the 2005-2021 Yamaha YZ125 and the 2020-2022 YZ125X, the "Size B" forged kit is the professional choice for riders performing their second or third top-end rebuild. At Moto-House MX, we know that as a 125cc cylinder sees more hours, the factory tolerances naturally loosen; this +0.01mm Size B kit provides the precise fitment needed to restore compression and eliminate piston rock in slightly worn bores.

Forged from high-performance aerospace aluminum alloy, the 8302DB is significantly stronger and more heat-resilient than stock cast pistons. It features a dual-ring configuration, which provides an exceptional secondary seal and improved heat dissipation—critical for 125cc engines that live in the high-RPM range. To ensure maximum longevity, Wossner applies a permanent molybdenum disulphide skirt coating that reduces friction and protects the cylinder wall. Precision-machined for 54.01mm bores, this 53.96mm piston ensures your YZ125 remains reliable and responsive on the track or trail.

Top Features: Wossner 8302DB Piston Kit

• Standard Size B (53.96mm): Precision-machined for 54.01mm bores to compensate for minor cylinder wear and restore factory-correct clearances.
• Dual Ring Configuration: Enhances cylinder sealing and cooling efficiency, helping to maintain consistent power throughout long motos.
• Aerospace-Grade Forging: Built to withstand high-RPM stress and thermal expansion far better than original cast alternatives.
• Low-Friction Skirt Coating: Permanent skirt treatment minimizes drag and piston rock for a quieter, smoother running engine and extended cylinder life.
• Optimized Lightness: Expertly machined to reduce reciprocating mass, ensuring the YZ125 retains its signature snappy throttle response.
• Complete Assembly: Includes the forged Size B piston, high-performance rings, wrist pin, and circlips.

Expert Pro-Tip: The Yamaha YZ125 is a high-performance machine that relies on perfect piston-to-wall clearance for maximum power. If your cylinder has 30-50 hours of ride time, stepping up to a Wossner Size B (53.96mm) is the standard expert method to bring your mechanical tolerances back to ideal levels. Always use a precision bore gauge to confirm your cylinder measures 54.01mm before installation. For a professional-grade rebuild, always install with a fresh top-end gasket kit and utilize premium 2-stroke oil to protect your forged assembly.

Fitment List

Year Range	Vehicle Brand	Vehicle Model
2005-2021	Yamaha	YZ125
2020-2022	Yamaha	YZ125X

Frequently Asked Questions

Q: When should I choose Size B instead of Size A?
A: Size B (53.96mm) is 0.01mm larger than Size A. It is used when the cylinder bore has worn slightly larger than the original 54.00mm, ensuring you maintain the tight seal necessary for peak horsepower.

Q: Is this kit compatible with the YZ125X off-road model?
A: Yes, this kit is specifically designed to fit both the YZ125 motocross bike and the YZ125X cross-country model within the specified year ranges.

Q: Do forged pistons require a different break-in process?
A: Forged pistons expand differently than cast pistons. It is recommended to perform 2-3 heat cycles, allowing the engine to cool completely between each, before riding at full race pace.

Recommended Pairings

• Complete Top End Gasket Set: Ensure a leak-free rebuild and maintain correct squish heights with fresh gaskets.
• Motul 800 2T Factory Line: High-performance lubricant to keep your new forged piston protected in high-RPM racing conditions.

Restore your machine’s power and precision today. Buy now at Moto-House MX!

Moto-House MX – Choice of Champions – is located in Clermont/Montverde, FL. The Best Motocross/UTV & ATV Shop in Central Florida. Stop by our local store in Montverde, FL or enjoy lightning-fast shipping across the USA.