just replace the bearing..they are not exspensive..
Unless you go to a Yamaha dealer...

Top bearing 25x47x15 tappered roller... bearing number "32005"
Bottom bearing 30x55x17 tappered roller.. bearing number "32006"

you will need to go to Yamaha for the bottom seal..

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Harley Davidson..
The easyest way to turn gasoline into noise with out the side efect of horse power...