Yamaha YZ426F(M)/LC Owner's Service Manual page 298

VALVES AND VALVE SPRINGS
NOTE:
After every lapping operation be sure to
clean off all of the compound from the valve
face and valve seat.
Valve spring
1. Measure:
2. Measure:
b Installed length
3. Measure:
4 - 32
Apply Mechanic's blueing dye (Dykem) to
the valve face.
Install the valve into the cylinder head.
Press the valve through the valve guide
and onto the valve seat to make a clear
pattern.
Measure the valve seat width again. If the
valve seat width is out of specification,
reface and relap the valve seat.
Valve spring free length a
Out of specification → Replace.
Free length (valve spring):
Intake:
39.42 mm (1.55 in)
<Limit>: 37.5 mm (1.48 in)
Exhaust:
41.80 mm (1.65 in)
<Limit>: 39.8 mm (1.57 in)
Compressed spring force a
Out of specification → Replace.
Compressed spring force:
Intake:
12.6 ~ 13.8 kg at 32.87 mm
(27.78 ~ 30.42 lb at 1.29 in)
Exhaust:
12.6 ~ 14.6 kg at 35.35 mm
(27.78 ~ 32.19 lb at 1.39 in)
Spring tilt a
Out of specification → Replace.
Spring tilt limit:
Intake:
2.5˚/1.7 mm (0.067 in)
Exhaust:
2.5˚/1.8 mm (0.071 in)
ENG