The OS 1.50 piston kit is a purpose-built aftermarket replacement designed for Yamaha YFM 400 FB and FW models produced from 1993 through 2012. This top-end kit concentrates on restoring reliable compression and power delivery after bore wear, making overhauls and performance restoration straightforward for trail, farm, and work-cycle use. With an oversize of +0.50 mm (OS 1.50), it brings worn bores back to proper diameter to reclaim factory-like throttle response and idle stability.

Engineered for durability in demanding riding conditions, this oversize piston kit targets the common wear points of the YFM 400 top end—piston rings, bore wear, and loss of compression that dulls performance under load. Restoring the bore size and compression helps maintain steady throttle response, consistent idle, and dependable performance when climbing hills, hauling loads, or navigating rough trails. It’s a practical upgrade path when the original top end shows signs of wear and is ready for a rebuild.

Comprehensive Vehicle Fitment

Product Specifications

OEM & Aftermarket Cross-Reference Information

What's Included

The box includes the core piston kit components required for a top-end rebuild on the YFM 400, specifically:

• Piston
• Piston Rings
• Wrist Pin
• Circlips

Note: Top-end gasket set, seals, and mounting hardware are not included in this kit and should be sourced separately as part of a complete overhaul.

Why Choose This Aftermarket Replacement?

• Direct-Fit Compatibility: Precisely sized for Yamaha YFM 400 FB/FW Manual models (1993–2012), enabling a straightforward top-end rebuild without guesswork.
• OEM Cross-Reference Potential: While OEM part numbers are not provided here, the OS 1.50 oversize design mirrors common Yamaha oversize specs used for top-end restoration on the YFM 400 family.
• Premium Materials & Design Intent: Engineered to withstand the heat and load of a 4-stroke top end, with an oversize bore intended to restore compression and power after bore wear.
• Performance Restoration: Restores throttle response, idle stability, and under-load power that’s typical of a well-tuned top end after rebuilding the cylinder.
• Cost-Effective Route: A practical alternative to OEM top-end components, offering reliable performance restoration at a lower price point.
• Trail- and Farm-Ready Durability: Built to handle the rough duty cycles of trail riding and everyday work use, returning consistent performance across a variety of conditions.

Installation Notes:

• Bore Inspection First: Inspect the cylinder bore for scoring or out-of-round wear; if worn, plan a proper overbore to accept the +0.50 mm oversize piston kit.
• Cleanliness is Critical: Thoroughly clean all oil passages, gallery openings, and seating surfaces to ensure optimal seal and ring seating during initial run-in.
• Rings and Orientation: Install new rings with correct orientation and ensure end gaps meet the manufacturer’s guidance for your specific bore size.
• Wrist Pin & Circlips: Install the wrist pin and circlips with care; verify that circlip grooves are clean and seated to prevent pin retention issues.
• Top-End Gasket Plan: Use a complete top-end gasket kit (not included) to ensure proper sealing between the cylinder, head, and intake/exhaust interfaces.
• Bearing Surfaces & Fasteners: Reuse only compatible fasteners per torque specs and replace any worn bearing surfaces as part of the full top-end refresh.
• Break-In Procedure: After installation, perform a proper break-in with light throttle for the first 20–30 minutes, avoiding high-load hammering until stable idle and smooth revs are confirmed.

Why Upgrade?

Running a worn top end on a Yamaha YFM 400 FB/FW can lead to compression loss, reduced throttle response, and uneven idle, especially under load. By upgrading to this OS 1.50 oversize piston kit, you restore bore size and compression, returning reliable power delivery and idle stability for everyday work and trail riding.