1991 Yamaha YFM250 MOTO-4 Wheel Bearing Inspection and Replacement Guide
Shop parts for a 1991 Yamaha YFM250 MOTO-4 ATV.
Solid Axle Design on the YFM250 MOTO-4: No CV Boots to Service
The 1991 Yamaha YFM250 MOTO-4 uses a solid rear axle design from an era before independent rear suspension with CV-jointed axles became standard on utility ATVs. On this machine, the rear axle is a fixed beam connecting both rear wheels through the axle housing, and there are no CV boots to service in the conventional sense. The rear drive system uses U-joints or simple shaft couplings rather than constant-velocity joints, and the wear maintenance most relevant to rear axle reliability on the YFM250 MOTO-4 is the wheel bearings and hub seals at each end of the rear axle housing.
How Wheel Bearings and Axle Seals Wear
Wheel bearings on the YFM250 MOTO-4 carry both the vertical load of the machine and the lateral forces generated during turns at each hub. The bearing is an assembly of steel rolling elements running between precision inner and outer races in a sealed or shielded housing. Over time, the rolling element and race surfaces develop micro-pitting from accumulated load cycles, and the grease inside the bearing breaks down from heat and contamination. The axle seal that keeps the bearing environment clean deteriorates as its rubber lip compound ages and hardens, eventually losing the sealing contact pressure needed to exclude water and grit.
Water contamination is the most common failure accelerant for rear wheel bearings on off-road machines like the YFM250 MOTO-4. An aged seal that no longer fully excludes water allows every creek crossing or wet riding session to introduce moisture and dissolved debris directly to the bearing. A bearing running contaminated grease wears at a significantly accelerated rate compared to one running clean compound, and the wear progresses with every subsequent ride once contamination has entered.
Recognizing Bearing Failure and Seal Damage
Wheel bearing wear on the YFM250 MOTO-4 shows up through two checks. The first is a play test: raise the rear of the machine on jack stands, grip the rear wheel at twelve and six o’clock, and try to rock it toward and away from the machine. Any perceptible play beyond normal tire flex indicates bearing clearance past the acceptable limit. Repeat gripping at three and nine o’clock and pushing in and out along the axle axis—axial play in this direction also indicates bearing wear. A spin test with the wheel in the air reveals grinding, roughness, or tight spots in the rotation that indicate pitting or surface damage in the bearing.
During riding, a groaning or humming noise from the rear end that changes with speed is a reliable early indicator of bearing degradation on the YFM250 MOTO-4. This sound is typically most audible at a specific speed range and may change character during turns as the load shifts between bearings. Grease or oil visible on the inner face of the rear wheel or at the hub seal area indicates a failed axle seal—a seal failure needs attention promptly to prevent further contamination from reaching the bearing behind it.
Tools and Parts for the Job
Rear wheel bearing replacement on the YFM250 MOTO-4 requires: floor jack and two jack stands; a bearing puller and driver set in sizes matching the rear hub bore on this machine; a seal driver or appropriate driver cup for the new seal; a torque wrench; clean shop rags; and, if the replacement bearing is an open design, the correct bearing grease. A bearing and seal kit sized specifically for the rear hub on the YFM250 MOTO-4 ensures correct dimensions and adequate load rating for this machine’s weight and use conditions. An undersized or mismatched bearing won’t provide the same service life as a correctly rated replacement.
Checking Bearing Condition
Raise the rear of the YFM250 MOTO-4 on jack stands with the wheel clear of the ground. Grip the wheel at twelve and six o’clock and rock it to check for radial bearing play. Repeat at three and nine o’clock to check for axial play. Spin the wheel by hand and feel and listen for grinding, roughness, or any point of resistance in the rotation. Document which wheel shows play or roughness before disassembly to confirm the correct side is being serviced.
If both wheels show similar amounts of play, both bearings may be at the same point in their wear cycle—utility ATVs that see symmetrical use loads tend to wear both rear hub bearings at a similar rate. Replacing both sides in the same service session avoids a repeat job shortly after the first repair and ensures the rear axle is in consistent condition across both wheel positions on the YFM250 MOTO-4.
Bearing and Seal Replacement
Remove the wheel from the YFM250 MOTO-4 and pull the hub assembly from the axle shaft. The axle seal is at the inboard face of the hub—pry it out carefully with a seal pick. With the seal removed, use the bearing puller to extract the old bearing from the hub bore, applying the pulling force to the outer race only. Clean the hub bore thoroughly, removing any corrosion, debris, or old grease. Press the new bearing into the bore squarely using the driver cup against the outer race—pressing on the inner race during installation damages the bearing before the first use. Install the new seal with the seal driver, pressing it flush with the hub face. Reassemble the hub on the axle shaft and torque all fasteners to spec before reinstalling the wheel.
Related Shopping Categories
Shop CV Joints for a 1991 Yamaha YFM250 MOTO-4 ATV.
Shop Axles for a 1991 Yamaha YFM250 MOTO-4 ATV.
Shop Lubricants & Solvents for a 1991 Yamaha YFM250 MOTO-4 ATV.
Shop Wheel Bearings for a 1991 Yamaha YFM250 MOTO-4 ATV.
Shop Skid Plates for a 1991 Yamaha YFM250 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.
