What to Service Instead of CV Boots on the 1991 Yamaha YFM350ER MOTO-4
Shop parts for a 1991 Yamaha YFM350ER MOTO-4 ATV.
Solid Axle Design on the YFM350ER MOTO-4: No CV Boots to Service
Unlike modern utility ATVs with independent rear suspension and CV-jointed axles, the 1991 Yamaha YFM350ER MOTO-4 drives the rear wheels through a solid axle housing—a fixed beam that keeps both rear wheels in a fixed geometric relationship to each other. This design uses shaft couplings or U-joints rather than constant-velocity joints, which means there are no CV boots to inspect or replace. The relevant wear service for rear axle reliability on the YFM350ER MOTO-4 is the wheel bearings and axle seals at the rear hub positions, which support the full load of the machine at the wheel and wear predictably over time and miles.
How Wheel Bearings and Axle Seals Wear
The rear hub bearings on the YFM350ER MOTO-4 are load-bearing components that take the full weight and cornering forces of the machine at each wheel mounting point. Inside the bearing, rolling elements run between inner and outer races in a precise geometric relationship that distributes load and allows smooth rotation. This relationship depends on the internal grease maintaining an adequate film between rolling surfaces. As the grease ages, contaminates, or depletes through a deteriorating seal, metal-to-metal contact between rolling elements and races begins—and each contact cycle accelerates the wear further.
The axle seal on the YFM350ER MOTO-4 is the first line of defense against contamination reaching the bearing. The seal lip contacts the rotating axle shaft and excludes water, mud, and grit from the bearing cavity. As the rubber ages and loses flexibility, the lip loses its sealing contact pressure and allows contamination past—particularly during water submersion. A seal that has failed allows every subsequent wet ride to drive contamination into the bearing, compressing what would otherwise be a multi-year wear timeline into a much shorter one.
Recognizing Bearing Failure and Seal Damage
Two checks reveal rear wheel bearing condition on the YFM350ER MOTO-4. Raise the rear end on jack stands and grip the rear wheel at twelve and six o’clock—any rocking play beyond tire flex indicates bearing clearance beyond the acceptable limit. Grip at three and nine o’clock and push in and out to check for axial play. Then spin the wheel by hand and listen for grinding, roughness, or any point where the rotation catches or feels rough. A bearing in serviceable condition allows smooth, quiet rotation throughout the full revolution.
A groaning or humming noise from the rear end during riding that changes with speed is a reliable sign of bearing wear in progress on the YFM350ER MOTO-4. Visible grease or oil at the rear hub seal location means the seal has failed and is no longer protecting the bearing from contamination. Address a failed seal promptly—continued riding with a failed seal allows each wet riding session to worsen the bearing condition faster than dry use alone would.
Tools and Parts for the Job
For rear wheel bearing service on the YFM350ER MOTO-4, stage: floor jack and jack stands; a bearing puller and driver set in the sizes needed for this hub; a seal driver cup; torque wrench; and shop rags. The replacement bearing must match the OEM inner and outer diameter, width, and load rating for the rear hub on this machine. A complete bearing and seal kit for the YFM350ER MOTO-4 ensures fitment and correct load rating. Using a proper bearing puller for extraction prevents hub bore damage that would affect how the new bearing seats and seals.
Checking Bearing Condition
To inspect rear wheel bearings on the YFM350ER MOTO-4, raise the rear end safely on jack stands. Grip the rear wheel and perform the rocking test at twelve and six o’clock for radial play, and at three and nine for axial play. Any detectable play beyond tire flex indicates a bearing at or past its service limit. Spin the wheel freely and listen for grinding or roughness across the full rotation. A bearing that feels rough or catches at a specific point has race or rolling element damage and needs replacement regardless of riding hours.
Check the opposite rear wheel in the same inspection session. Utility ATVs that accumulate consistent off-road and work use often wear both rear hub bearings at a similar rate, and finding one side at or past the service limit frequently means the other is close behind. Addressing both positions at once saves a repeat disassembly and ensures balanced rear axle condition on the YFM350ER MOTO-4.
Bearing and Seal Replacement
Pull the rear wheel from the YFM350ER MOTO-4 and remove the hub from the axle shaft. Extract the old axle seal carefully with a seal pick, working around the lip without scoring the bore. Position the bearing puller against the outer race of the old bearing and press or pull it free. Clean the hub bore completely. Press the new bearing squarely into the bore using the correct driver cup against the outer race only—force applied to the inner race pre-loads and damages the bearing. Install the new seal using the driver, pressing it flush and square. Reassemble the hub on the YFM350ER MOTO-4’s axle shaft, torque all fasteners to spec, and reinstall the wheel.
Related Shopping Categories
Shop CV Joints for a 1991 Yamaha YFM350ER MOTO-4 ATV.
Shop Axles for a 1991 Yamaha YFM350ER MOTO-4 ATV.
Shop Lubricants & Solvents for a 1991 Yamaha YFM350ER MOTO-4 ATV.
Shop Wheel Bearings for a 1991 Yamaha YFM350ER MOTO-4 ATV.
Shop Skid Plates for a 1991 Yamaha YFM350ER MOTO-4 ATV.
Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your ATV's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.
