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 InspectionWiring Diagram (Other than US, CA and CAL with KIBS Models)
ZX1000KC
ZX1000KD
...
Before Servicing
Before starting to perform an inspection service or carry out a disassembly
and reassembly operation
on a motorcycle, read the precautions given below. To facilitate actual
operations, notes, illustrations,
photographs, cautions, and detailed descriptions have been included in each
chapter w ...
Inquiries to Rider
DFI Diagnosis Flow Chart
Each rider reacts to problems in different ways, so it is important to
confirm what kind of symptoms
the rider has encountered.
Try to find out exactly what problem occurred under exactly what
conditions by asking the rider;
knowing this information may h ...