Wossner 8321D400 Piston Kit - 2022-2026 Yamaha YZ125 YZ125X +4.00mm Oversize

Wossner 8321D400 Standard Series Forged 2-Stroke Piston Kit - +4.00mm Oversize

Push the limits of your small-bore engine with the Wossner 8321D400 Piston Kit. Specifically designed for the latest generation 2022-2026 Yamaha YZ125 and YZ125X models, this +4.00mm oversize forged kit is the ultimate choice for custom big-bore builds or for salvaging cylinders that require extensive boring and replating to 58.00mm. At Moto-House MX, we know that serious racers and builders trust Wossner’s aerospace-grade forgings to provide the structural integrity required when increasing displacement and power.

Forged from premium aluminum alloy, the 8321D400 features a performance-driven single-ring design to minimize internal friction and maximize peak RPM. Despite the significant jump in diameter to 58.00mm, Wossner utilizes advanced machining to keep the piston lightweight, ensuring your YZ retains the rapid throttle response and "flickability" that defines the 125cc class. The permanent skirt coating is vital in these larger-bore applications, reducing internal drag and piston rock for a quieter running engine and a reliable break-in. This 57.95mm piston is precision-matched for 58.00mm bores, offering a durable, high-performance solution for your custom Yamaha build.

Top Features: Wossner 8321D400 Piston Kit

• +4.00mm Oversize (57.95mm): Engineered specifically for 58.00mm bores, allowing for increased displacement and the repair of heavily damaged cylinders.
• Single Ring Design: Minimizes friction and drag, allowing for the extreme RPMs and high-end power required for competitive motocross.
• Aerospace-Grade Forging: Significantly stronger than stock cast pistons, offering superior fatigue resistance for high-stress racing conditions.
• Low-Friction Skirt Coating: Permanent molybdenum disulphide coating protects the cylinder wall and ensures smooth operation under high thermal loads.
• Optimized Lightness: Machined to remove excess mass while reinforcing the crown and skirt for professional-grade durability.
• Complete Assembly: Kit includes the forged piston, high-performance ring, wrist pin, and circlips.

Expert Pro-Tip: Moving to a +4.00mm oversize is a major modification that requires the cylinder to be professionally bored and replated to a 58.00mm finished size. This effectively turns your engine into a "big bore" 125, often requiring changes to your jetting and power valve clearance. When installing the 8321D400, always double-check your ring end gap and ensure the cylinder head is modified to accommodate the larger 58mm bore. Pairing this build with a fresh top-end gasket kit and high-quality 2-stroke premix oil will ensure your big-bore Yamaha performs at its peak.

Fitment List

Year Range	Vehicle Brand	Vehicle Model
2022-2026	Yamaha	YZ125 (Standard, Monster Energy, 50th/70th Anniversary)
2023-2026	Yamaha	YZ125X (Cross Country)

Frequently Asked Questions

Q: Is this a drop-in piston for a stock bike?
A: No, this is a +4.00mm oversize piston. Your cylinder must be bored and replated to exactly 58.00mm to use this piston kit.

Q: Will this increase my engine's displacement?
A: Yes, boring your cylinder to 58.00mm will increase the displacement beyond the stock 125cc, providing more torque and potential power, though it may change your racing class eligibility.

Q: Does this kit include the gasket set?
A: No, the kit includes the forged piston, ring, wrist pin, and clips. A complete top-end gasket set is sold separately and is required for a professional rebuild.

Recommended Pairings

• Complete Top End Gasket Set: Ensure a professional, leak-free seal for your custom engine build.
• Yamalube 2-R Premix Oil: The gold standard for keeping high-performance Yamaha 2-strokes running clean and protected.

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.