OS 1.00 oversize piston kit for the Yamaha YFM 350 family, including Big Bear, Raptor, Bruin, Grizzly, is designed to restore compression and reliable performance after bore wear. This aftermarket replacement kit provides a complete top-end rebuild path, including the essential components you need to bring worn cylinders back to life. The 1.00 mm oversize bore increases capacity while preserving proper piston-to-cylinder clearance for dependable operation across multiple model years. The kit leverages forged aluminum construction for strength and durability, helping you regain responsive throttle and steady power on the trail or on the farm.

Unlike OEM-only parts, this OS 1.00 piston kit is tailored to restore compression and throttle response after wear, delivering a reliable rebuild option that keeps you rolling. The included top-end gasket kit supports a proper seal during break-in, so you can trust the rebuild to perform consistently when conditions get tough on dirt, mud, or uneven terrain. Built for durability in demanding environments, this aftermarket option is a practical, cost-conscious choice for trail riders and agricultural users alike.

Comprehensive Vehicle Fitment

Note: Fitment reflects cross-reference data and Yamaha designations. Always verify against your exact engine bore and model year before ordering.

Product Specifications

Cross-reference and OEM notes confirm the OS 1.00 oversize option aligns with Yamaha’s standard part numbers for the YFM 350 family, providing a direct pathway from OEM references to aftermarket replacement.

OEM & Aftermarket Cross-Reference Information

These cross-reference entries reflect Yamaha’s documented piston sizes and the availability of an OS 1.00 oversize alternative from aftermarket suppliers.

What's Included

This OS 1.00 piston kit includes everything you need for a complete top-end rebuild on the YFM 350 family:

• Piston
• Piston Rings
• Wrist Pin
• Circlips
• Top-End Gasket Kit

Related parts not included: cylinder head, crankshaft assembly, camshaft, or other engine components are not included with this kit.

Why Choose This Aftermarket Replacement?

• Direct-Fit Compatibility: Precisely sized OS 1.00 kit designed to match Yamaha YFM 350 engines without modification for a straightforward rebuild.
• OEM Cross-Reference: Maintains compatibility with Yamaha OEM references while offering a readily available aftermarket alternative.
• Premium Materials (Forged Aluminum): Forged aluminum piston delivers strength and durability under load, helping resist wear and scuffing in a high-rev top end.
• Oversize Bore for Worn Bores: +1.00 mm oversize restores compression and power on engines with bore wear, extending service life between major overhauls.
• Complete Top-End Kit Included: The package includes piston, rings, wrist pin, circlips, and a top-end gasket kit for a thorough rebuild.
• Reliable Performance for Trail & Farm: Built to withstand the rigors of off-road riding and field work, returning dependable throttle response and power.

Installation Notes:

• Before install, thoroughly clean the cylinder bore and verify bore wear; measure with a bore gauge and compare to the service limits for your model year.
• Install with a fresh, dedicated top-end gasket kit and new circlips; ensure all circlips seat fully into their grooves to avoid wrist pin failure.
• Lubricate the piston skirts, rings, and wrist pin with assembly lube or light engine oil before assembly to prevent dry friction on initial startup.
• Use a ring compressor to seat the piston rings evenly into the bore; confirm ring gaps and orient the piston with correct intake/exhaust orientation as per the service manual.
• Clean all mating surfaces on the cylinder head and cylinder to ensure an even seal with the new gasket kit; remove oil and debris before reassembly.
• After assembly, perform a proper break-in procedure: keep RPMs moderate for the first 20–30 minutes and avoid sustained high RPMs until the rings seat fully.

Why Upgrade?

Running a worn piston from compression loss or ring wear can lead to reduced power, higher oil consumption, and degraded throttle response, especially under load on trails or field work. Replacing with this OS 1.00 piston kit restores compression, power delivery, and overall reliability, giving your Yamaha YFM 350 a fresh top end that’s ready for tough conditions.