Yamaha 115-225 HP Outboards Service Manual, Page 235Get this manual
I 1.1 -ELEC STARTER MOTOR
4) Measure the commutation mica undercut depth
If the undercut depth is less than the specified minimum, cut it with hacksaw blade, and then remove all particles of mica and metal by compressed airStandard undercut depth: 0.65 mm (0.026 in) Minimum undercut depth: 0.20 mm (0.008 in) Armature 1) Test to check that the commutator is not earthed to the shaft or laminations, and if such continuity is found, replace the armature