Athena Big Bore Kit for the Yamaha YZ250 & YZ250X

The Athena Big Bore Kit for the Yamaha YZ250 delivers a significant performance boost with a larger displacement and improved cylinder design. This complete top-end kit includes a nickel–silicon carbide–coated cylinder, forged piston, rings, gaskets, and all required components for installation. Engineered for durability and consistent power delivery, the Athena kit is a proven upgrade for riders looking for stronger acceleration, increased torque, and reliable performance in demanding off-road conditions.

Features:

• Complete big bore top-end kit from Athena

• Includes plated cylinder, forged piston, rings, pin, clips, and gaskets

• Nickel–silicon carbide-coated cylinder for wear resistance and heat stability

• Designed for improved torque and throttle response

• Precision-engineered components for reliable sealing and fitment

• Manufactured in Europe to OEM-level quality standards

• Direct replacement — no cylinder re-plating required

Fitment:

• 2003–2026 – Yamaha YZ250 Two Stroke

• 2016–2026 – Yamaha YZ250X Two Stroke

Fitment Notes:

• The cylinder head and base gaskets contained in the kit are specifically developed for Athena cylinders kits. Whenever the cylinder kit is disassembled, replace the gaskets with the specific ATHENA GASKET KIT: DO NOT use OE replacement gaskets.
• Jetting: Because this kit significantly increases displacement and air intake, you must re-jet your carburetor to prevent the engine from running lean, which can cause overheating or engine failure.
• Exhaust: To get the full potential gains of this kit, a high-flow aftermarket exhaust is highly recommended. Check your local laws for restrictions.
  

Specifications:

• Bore Diameter: 72mm  **Stock bore is 66.4mm

• Displacement: 293

• Compression Ratio: 15:1

• Stroke: Stock crank shaft is retained with a 72mm stroke

Find the replacement gasket kit for the
YZ250 / YZ250X Big Bore Kit:  CLICK HERE

Part Number: P400485100042