OS 1.50 Piston Kit for Yamaha YFM 350 Big Bear, Raptor, Bruin, and Grizzly is a purpose-built top-end rebuild that increases displacement with a +1.50 mm oversize bore. It’s designed to restore compression and deliver stronger mid-range torque and quicker throttle response on the trail or farm, while staying true to Yamaha’s original engineering intent when used with the proper supporting parts. This aftermarket replacement is structured to offer a straightforward, reliable rebuild experience without OEM pricing. The kit is compatible across the YFM 350 platform, making it a versatile upgrade for multiple model lines. The OS 1.50 kit combines robust forged aluminum performance with an exacting fit for the YFM 350 family, addressing wear on the piston, rings, and related top-end components. By restoring compression and displacement, it helps recover lost low- and mid-range power and improves throttle feel under load. When paired with proper rings, wrist pin, and circlips, this oversize kit provides dependable operation for rugged trail work or farm duty, delivering lasting performance through the life of the rebuilt engine.

Comprehensive Vehicle Fitment

This OS 1.50 piston kit aligns with Yamaha YFM350 engines across Warrior, Big Bear, Bruin, Grizzly, and Raptor lineages, covering a broad range of 4-stroke, air-cooled configurations used in trail and utility models.

Manufacturer	Models / Series	Years / Details
Yamaha	YFM350 Warrior (YFM350X)	1987–2004
Yamaha	YFM350 WT Big Bear	1989–1999
Yamaha	YFM350 Bruin	Various years (4-stroke, air-cooled)
Yamaha	YFM350 Grizzly	2007–2014 (Grizzly 350 family; 4x4 Auto and variants)
Yamaha	YFM350 Raptor	2005–2013
Yamaha	YFM350 Grizzly IRS 4x4 Auto	2007–2011

Note: fitment reflects how the YFM350 platform has been used across these model lines and years; always verify bore compatibility and supporting components during installation.

Product Specifications

Attribute	Details
OEM Reference Part Numbers	17211-HP0-A00, 17254-HN1-000, 17254-HP0-A00
Type	Piston Kit
Manufacturer	Aftermarket Replacement
Application	Yamaha YFM 350 Big Bear, Raptor, Bruin, Grizzly 4-stroke engines
Material	Forged aluminum
Bore Oversize	+1.50 mm (approx. +0.059 in)
Engine Type	4-stroke
Piston Style	OS oversize (piston kit)

OEM & Aftermarket Cross-Reference Information

Part Number	Brand / Type	Cross-Reference Notes
17211-HP0-A00	OEM Yamaha	Original equipment reference; used for cross-checking OS oversize replacements and ensuring compatibility with rings/wrist pin/circlips
17254-HN1-000	OEM Yamaha	OEM reference; serves as a cross-check point for aftermarket oversize kits
17254-HP0-A00	OEM Yamaha	Alternative OEM reference; verify bore compatibility with the +1.50 mm oversize kit

What's Included

A complete OS 1.50 piston kit ships with the core components needed for a top-end rebuild.

• Piston assembly
• Rings set
• Wrist Pin
• Circlips

Note: related top-end items such as a gasket kit are not included and are typically sold separately.

Why Choose This Aftermarket Replacement?

• Direct-Fit Compatibility: Engineered to drop in on Yamaha YFM350 top ends, preserving the intended clearance, pin diameter, and ring grooves when used with the proper supporting parts.
• OEM Cross-Reference: Uses established OEM references for reliable verification during ordering and reassembly.
• Premium Materials: Forged aluminum construction provides strength and durability under load, helping resist knock and wear in rugged use.
• Displacement Upgrade: Oversize bore (+1.50 mm) increases displacement to boost mid-range torque and throttle response.
• Performance Restoration: Restores lost power from worn rings, piston skirt wear, and compression leaks in the top end.
• Trail- and Farm-Ready Reliability: Built to endure off-road conditions, with precise machining and compatibility with standard wear items used in field maintenance.

Installation Notes:

• Verify the cylinder bore is properly prepared for a +1.50 mm oversize piston, including honing or boring per manufacturer guidelines.
• Measure piston ring end gaps and ensure correct ring orientation before installation; use the correct gap spec for the OS size and compression target.
• Lubricate the piston skirt and rings with clean engine oil before assembly to reduce initial wear on start-up.
• Install wrist pin and circlips with the piston oriented to match the engine’s intake/exhaust timing and rod orientation.
• Check that all mating surfaces (cylinder head, base gasket, and head gasket) are clean, undamaged, and compatible with the oversize bore.
• Follow the engine service manual for general top-end torque guidance and final assembly checks; verify oil passages and cooling clearance after assembly.

Why Upgrade?

Operating with a worn or undersized piston kit can lead to compression loss, reduced power, and accelerated wear in the top end, especially under load on trails or farms. Upgrading to the OS 1.50 piston kit restores compression and displacement, delivering consistent power delivery, improved mid-range torque, and renewed reliability for demanding uses. The result is a more responsive, durable engine that performs like new under practical conditions.