2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 203Get this manual
MIDDLE GEAR
4.Inspect: O-ring Damage ReplaceBearings Pittingdamage Replace5.Check: U-joint movement Roughness Replace U-joint
MIDDLE DRIVE AND DRIVEN GEAR SHIM SELECTION When the drive and driven gear, bearing housing assembly andor crankcase replaced, be sure to adjust the gear shim 11.Select: Middle drive gear shim Middle driven gear shim 2