Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 69Get this manual
3Measure the armature shaft for deflectionIf the armature has greater than 0.002 in(0.05 mm) deflection, replace armature 5Measure the mica undercut of the commutatorClean out and undercut mica and build up of brush material to depth of 0.0030.008 in(0.20.8 mm)
aArmatureV-BlockDial Indicator Gauge
aCommutator DepthFile (or use commutator cutter)
4Measure the commutator diameterReplace if not within 1.141.18 in(29.030.0 mm) diameter