Athena S4C04750005A Piston Kit (Size A) — 2002-2025 Yamaha YZ85 85cc 2-Stroke

Athena — Factory Quality Precision, Superior Thermal Resistance, and Proven Dyno Performance

When it’s time to refresh your Yamaha YZ85’s top end, choosing a piston that matches the exacting tolerances of your OE cylinder is critical for maintaining peak horsepower. The Athena S4C04750005A Piston Kit is a premium "Size A" (47.45 mm) cast replacement designed specifically for 85cc 2-stroke engines. Engineered as a high-performance alternative to stock, this kit ensures your mini-racer retains the "hit" and reliability required for competitive motocross.

Constructed from a specialized 4032 aluminum alloy with a high silicon content, this domed piston offers exceptional resistance to the intense mechanical and thermal stresses of high-RPM racing. Athena subjects every piston design to rigorous engine dynamometer testing under progressive stress loads, followed by extensive track validation, ensuring that your YZ85 performs at its best even in the most extreme heat and racing conditions.

• Cast from high-performance 4032 aluminum alloy for maximum structural integrity.
• High silicon content guarantees OE-level reliability with superior heat dissipation.
• "Size A" (47.45 mm) bore specifically tailored for fresh or low-hour OE cylinders.
• Extensively tested on engine dynos and track-validated for extreme off-road use.
• Complete kit includes the domed piston, rings, wrist pin, and circlips for a full top-end rebuild.

Moto-House MX Pro Tip:

Top-end maintenance is the lifeblood of a 2-stroke YZ85. When installing this Athena Piston Kit, always measure your cylinder bore to ensure "Size A" provides the correct piston-to-cylinder clearance. We also recommend thoroughly cleaning the power valve assembly while the cylinder is off. Using a high-quality 2-stroke premix oil during the break-in period will help seat the rings perfectly, ensuring maximum compression and a longer life for your new top end.

Year	Make	Model Fitment
2002-2025	Yamaha	YZ85 / YZ85LW (Large Wheel)

FAQ Questions

Q) What does "Size A" mean for this piston?
A) "Size A" (47.45 mm) is the standard size for a new or well-maintained stock cylinder. If your cylinder has significant wear, you may need a Size B or to have the cylinder replated.

Q) Does this kit include everything needed for a top-end change?
A) This kit includes the piston, ring, wrist pin, and circlips. You will still need a top-end gasket kit (sold separately) to complete the installation.

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