Wheel bearings are not replaceable on the lower stock Yamaha wheels (The inner & outer wheels that bolt to the skidframe). On the replacement Kimpex wheels, the bearings are replaceable.
The upper two wheels have replaceable bearings (Just replaced them).
The 2 outer big wheels at the back that adjust track tension also do not have replaceable bearings. Just hope you don't have to replace those, the wheel / bearing unit is $78 each. The inner big wheel at the back does have a replaceable bearing.
I've become somewhat of an expert on Attak idler wheels. I've replaced 7 with failed bearings and 3 that delaminated. Two of the delaminated wheels were covered under warranty.
My Attak has 4200 miles, my 2005 SkiDoo has 5300 miles, and I've never replaced a wheel. Yamaha needs to get this figured out.