Yanmar 3YM30, Service Manual

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5. Inspection and servicing of basic engine parts
116R1
5.3 Piston and piston pins
Pistons are made of a special light alloy with superior
thermal expansion characteristics, and the top of the
piston forms a combustion chamber.
IMPORTANT :
Piston shape differs among engine models. If any
incorrect piston is installed, combustion performance
will drop. Be sure to check the applicable engine
model identification mark on the piston to insure use
of the correct part.
5.3.1 Piston
(1) Piston head and combustion surface
Remove the carbon that has accumulated on the piston head and combustion surface, taking care not to
scratch the piston. Check the combustion surface for any damage.
(2) Measurement of piston outside diameter/
inspection
1) Replace the piston if the outsides of the piston or
ring grooves are worn.
2) Measure the outside diameter in the position of
22mm from the piston bottom in the right angle
direction of the piston pin.
If the piston outside diameter exceed the limit, replace the piston with new one.
Selective pairing of cylinder and piston
Piston must be paired with cylinder according to the below table. The size mark of a piston is shown on
the top surface of the piston and the size mark of a cylinder block is shown on the non-operating side of
the cylinder block. The service parts of pistons are provided.
ID marks for piston size
ML
MS
Model ID marks for engine model
3YM30 76K
3YM20/2YM15 70
Piston outside diameter
Model Standard Limit
Clearance between
piston and cylinder
3YM30
75.965-
75.975
75.920 0.035-0.055
3YM20
2YM15 69.970-
69.980
69.925 0.030-0.050
1st compression ring
2st compression ring
Oil ring
I.D. mark
Piston
Piston pin
Circlip
(Piston outside diameter)
Measurement
position
22 mm