2004-2009 Honda TRX450R/TRX450ER Service Manual, Page 173Get this manual
COOLING SYSTEM INSTALLATION Install the thermostat into the cylinder head with the bleed hole facing up
Install new O-ring into the groove in the thermostat cover
Install the thermostat cover onto the cylinder head and tighten the three bolts securelyFill and bleed the cooling system (page 9-7)