5. Inspection and servicing of basic engine parts
114
5.2.6 Measuring top clearance
(1) Place a high quality fuse (
Ø
1.5 mm, 10 mm long) in three positions on the flat part of the
piston head.
(2) Assemble the cylinder head gasket and the cylinder block and tighten the bolts in the
specified order to the specified torque.
(3) Turn the crank, (in the direction of engine revolution), and press the fuse against the
piston until it breaks.
(4) Remove the head and take out the broke fuse.
(5) Measure the three positions where each fuse is broken and calculate the average.
5.2.7 Intake and exhaust rocker arms
The wear of rocker arm and rocker arm bushing may change opening/closing timing of the valve, and
may in turn affect the engine performance according to the extent of the change.
(1) Rocker arm shaft and rocker arm bushing
Measure the outer diameter of the shaft and the inner-
diameter of the veering, and replace if wear exceeds
the limit.
Replace the rocker arm bushing if it moves and replace the entire rocker arm if there is no tightening
clearance.
(2) Valve spring
Check the rocker arm spring and replace it if it is corroded or worn.
(3) Rocker arm and valve top retainer wear
(4) Inspect the contact surface of the rocker arm and replace it if there is abnormal wear or
flaking.
Model
Top clearance (mm)
3YM30
0.747-0.891
3YM20
0.697-0.841
mm
Standard
Limit
Outside dia of intake and
exhaust rocker arm shaft.
11.966-11.984
11.94
Intake and exhaust rocker
arm bushing inside dia.
12.000-12.020
12.07
Rocker arm shaft and
bushing clearance at
assembly
0.016-0.054
0.13
Fuse wire
(Rocker arm hole diameter)
Summary of Contents for 3YM20
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