2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 330Get this manual
ELECTRIC STARTING SYSTEM
STARTER MOTOR INSPECTION 1.Inspect: Commutator Dirty Clean it with #600 grit sandpaper2.Measure: Commutator diameter Out of specification Replace the starter motorOutside diameter: 28 mm (1.10 in) Wear limit: 27 mm (1.06 in) 3.Measure: Mica undercut Out of specification Scrape the mica using hacksaw bladeMica undercut: 0.7 mm (0.03 in) NOTE: Scrape the mica to the proper measurement using hacksaw blade which has been grounded to fit the commutator