1999-2004 Honda TRX400EX FourTrax Service Manual, Page 101Get this manual
CYLINDER HEADVALVE
CYLINDER HEAD ASSEMBLY INSTALLATION-
Blow through the oil passage in the cylinder head with compressed airInstall the valve spring seatsInstall new stem sealsLubricate the valve stem sliding surface with molybdenum oil solutionlnsett the valve into the guide while turning it slowly to avoid damage to the stem sealr