Yamaha F100B F100C Outboards Factory Service Manual, Page 317Get this manual
Starter motor 3Measure the commutator undercut aReplace the armature if out of specification
Checking the brushes
1Measure the brush lengthReplace the brush assembly if out of specification
Commutator undercut limit a: 0.2 mm (0.01 in) 4Check the armature for continuityReplace if out of specifications
Brush length limit a: 9.5 mm (0.37 in) 2Check the brush holder assembly for continuityReplace if out of specifications