2004 Official factory service manual for Yamaha YFZ450S ATV Quad., Page 166Get this manual
VALVES AND VALVE SPRINGS
fApply Mechanic's blueing dye (Dykem) to the valve facegInstall the valve into the cylinder headhPress the valve through the valve guide and onto the valve seat to make clear patterniMeasure the valve seat width againIf the valve seat width is out of specification, reface and relap the valve seatLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL
10.Measure: valve spring free length Out of specification ReplaceValve spring free length Intake 37.03 mm (1.46 in) Limit: 35.17 mm (1.38 in) Exhaust 37.68 mm (1.48 in) Limit: 35.79 mm (1.41 in)