To undo, bend out the upper facing tab washer tabs, undo the top nut using this tool, remove the tab washer then undo the lower nut using this tool. To refit, reverse this operation using a torque wrench on the tool to torque up the lower nut (to no more than 40Nm initially to settle the bearings then slacken off and retighten to generally 15-18Nm), fit the tab washer then finger tighten the top nut and finally tighten the top nut using the tool so that the upper tabs align with the next available notch in the top nut.
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