If any is worn down to the service limit, replace the brush assy.
Starter Motor Brush Length [B] Standard: 12.05 mm (0.4744 in.) Service Limit: 6.55 mm (0.258 in.)

Starter Motor Assembly
Commutator Cleaning and InspectionTiming Rotor Installation
Install the timing rotor [A] on the crankshaft [B] with their
teeth [C] aligned.
Holding the timing rotor with the flywheel & pulley holder
and tighten the bolt.
Torque - Timing Rotor Bolt: 39 N·m (4.0 kgf·m, 29 ft·lb)
Special Tool - Flywheel & Pulley Holder: 57001-1605
I ...
Starter Clutch Inspection
Remove:
Torque Limiter Cover (see Torque limiter Removal)
Turn the torque limiter [A] by hand. The torque limiter
should turn forward freely [B], but should not turn backward.
If the starter clutch does not operate as it should or if it
makes noise, go to the next step.
Disassem ...
Rider’s Seat Installation
Insert the tabs on the front of the
rider’s seat into the slots in the frame
and secure the seat with mounting
bolts.
Install the seat cover and bolts.
A. Rider’s Seat
B. Tabs
C. Slots
D. Stays ...