Dismantle the fixed bolts with the hex wrench, then the tension wheel and guide wheel.

There are differences between tension wheel and guide wheel. You need to mind it in case of wrong installation.

Clean all the corner of the pulley, then put some butter on the gap between the pulley and bearing.

Left is tension wheel, right guide wheel.

After maintainance, put the pulley back to the rear  derailleur.

Guide wheel(pay attention to the direction, do not make mistakes.)
Tension wheel(pay attention to the direction, do not make mistakes.)

When you fix the pulley, remember to control your strength(2.5-5.0 N.m) avoid falling of the screws.

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