ProX Piston Kit (66.36mm B / 66.37mm C) 01.2321.B / 01.2321.C - Yamaha YZ250 / YZ250X / Suzuki RM250 1999-2026

Product Description

Refresh your top end with the ProX Piston Kit (MPNs 01.2321.B and 01.2321.C), manufactured in Japan as a direct OEM-style replacement for high-performance 250cc 2-stroke engines. Available in two precision size options—Size B (66.36mm) and Size C (66.37mm)—these pistons allow you to dial in exact piston-to-cylinder clearance on standard 66.40mm bore cylinders with broken-in or wearing plating. Each kit includes a piston, high-performance ring set, wrist pin, and circlips required for a complete top-end overhaul.

Cast from a high-silicon-content aluminum alloy, ProX pistons meet or exceed original equipment specifications for thermal expansion and durability. CNC-machined to extremely close tolerances, each piston features a low-friction Moly coated skirt that reduces drag and protects the cylinder wall during cold starts and high-RPM operation. Whether you are servicing your practice bike or prepping for race day, ProX delivers OEM performance and long-lasting reliability.

Top Features: ProX Piston Kit

• Manufactured in Japan and designed as a direct OEM replacement top-end rebuild kit.
• Available in Size B (66.36mm) and Size C (66.37mm) options to precisely match cylinder plating wear.
• Cast from a high-silicon-content aluminum alloy to meet or exceed OEM thermal and strength specifications.
• CNC-machined to extremely close tolerances with a low-friction Moly coated skirt.

Please review the detailed compatibility data below to ensure an exact fitment match for your motorcycle before finalizing your purchase.

Fitment List

Frequently Asked Questions

Q: What is the difference between the B and C piston size options?
A: ProX manufactures pistons in slight increment sizing (A, B, C) to match cylinder wear on plated cylinders. Size B measures 66.36mm and Size C measures 66.37mm to compensate for normal cylinder plating wear and restore ideal piston-to-wall clearance on a standard 66.40mm bore.

Q: Does each option include all necessary hardware for a top-end piston replacement?
A: Yes, each ProX kit includes the selected piston, piston rings, wrist pin, and circlips.

Expert Pro-Tip: Always measure your cylinder with a dial bore gauge prior to selecting your piston size grade. When installing new piston rings on a 2-stroke, check ring end gap inside the cylinder bore and make sure the ring locating pins on the piston align cleanly with the ring gaps to prevent snagging on exhaust or intake ports during assembly.

Recommended Pairings:

• Yamaha YZ250 & YZ250X Performance Parts
• Suzuki RM250 Performance Parts
• Yamaha 2-Stroke Performance Parts

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