1999-2004 Honda TRX400EX FourTrax Service Manual, Page 150Get this manual
CRANKCASETRANSMISSIONCRANKSHAFT Check the gears for freedom of movement or rotation on the shaftEngage the mainshaft and countershaft gears
Be sure to install the thrust washers
If removed, insall thecrankshaftbalancer (page 11-11), Apply oil to the countershaft oil seal lipInstall the mainshaft and countershaft assemblies as set into the left crankcase