Cylinder Works Big Cylinder Works 72mm High-Output Big Bore Kit (422-21009K / 21009-K01) | Yamaha YZ250 (1999-2025)

Complete high-performance big bore cylinder kit engineered to significantly increase displacement, torque, and peak horsepower without requiring case modification. Features a premium cast aluminum cylinder with an enlarged 72.00 mm bore (+5.6mm over stock) finished in a durable nickel silicon carbide (NSC) electroplate. Paired with a matched, high-strength forged big bore piston featuring a hi-tech Molybdenum Disulphite skirt coating for accelerated break-in and extended service life, alongside a complete top-end gasket kit.

Specs & Details

• Manufacturer Part Number: 422-21009K
• Alternative Part Number: Cylinder Works 21009-K01
• Kit Type: Complete Big Bore Cylinder, Piston, and Gasket Kit
• Bore Size: Big Bore (72.00 mm / +5.6mm over stock)
• Material & Construction: Nickel silicon carbide plated aluminum cylinder with a T-5 heat-treated forged piston

Two-Stroke Vehicle Fitment

TSO Garage Note

The ultimate bolt-on engine modifier for YZ250 riders hunting for serious, tractive power across the entire RPM range. Bumping displacement up to a true 72.00mm big bore transforms the power delivery with hard-pulling torque down low and increased peak horsepower up top. Whether you are lugging through technical single-track trails, hooking up in deep sand dunes, or charging down long motocross straights, the extra bottom-end grunt pulls taller gearing with ease. Because the cylinder drops straight into the stock cases with no machine work required, it's an efficient way to unlock a whole new level of performance during a standard top-end rebuild.