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6. CYLINDER HEAD/CYLINDER/PISTON
6-0
BET & WIN 5 0
6
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CYLINDER HEAD/CYLINDER/PISTON
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SERVICE INFORMATION........................................................ 6-1
TROUBLESHOOTING.............................................................. 6-1
CYLINDER HEAD.................................................................... 6-2
CYLINDER/PISTON................................................................. 6-4
6
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6. CYLINDER HEAD/CYLINDER/PISTON
6-1
BET & WIN 5 0
SERVICE INFORMATION
GENERAL INSTRUCTIONS
• The cylinder head, cylinder and piston can be serviced with the engine installed in the frame.
• Before disassembly, clean the engine to prevent dust from entering the engine.
• Remove all gasket material from the mating surfaces.
• Do not use a driver to pry between the cylinder and cylinder head, cylinder and crankcase.
• Do not damage the cylinder inside and the piston surface.
• After disassembly, clean the removed parts before inspection. When assembling, apply the
specified engine oil to movable parts.
SPECIFICATIONS
Standard (mm)
Service Limit (mm)
Item
B&W50
B&W50
Cylinder head warpage
0.10
Piston O.D.(5mm from bottom of piston
38.970_ 38.955
38.90
Cylinder-to- piston clearance
0.03_ 0.07
0.10
Piston pin hole I.D.
12.002_ 12.008
12.03
Piston pin O.D.
11.994_ 12.0
11.98
Piston-to-piston pin clearance
0.002_ 0.014
0.03
Piston ring end gap (top/second)
0.10_ 0.25
0.40
Connecting rod small end I.D.
17.005_ 17.017
17.03
Cylinder bore
39.0_ 39.025
39.05
TORQUE VALUES
Cylinder head bolt 14.7_ 16.66N-m
Exhaust muffler joint lock nut 9.8_ 13.72N-m
Exhaust muffler lock bolt 29.4_ 35.28N-m
Spark plug 10.78_ 16.66N-m
TROUBLESHOOTING
Compression too low, hard starting
or poor performance at low speed Abnormal noisy piston
• Leaking cylinder head gasket • Worn cylinder and piston
• Loose spark plug • Worn piston pin or piston pin hole
• Worn, stuck or broken piston and piston rings • Worn connecting rod small end bearing
• Worn or damaged cylinder and piston
Compression too high, overheating
or knocking Abnormal noisy piston rings
• Excessive carbon build-up in cylinder head • Worn, stuck or broken piston rings
or on piston head • Worn or damaged cylinder
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6. CYLINDER HEAD/CYLINDER/PISTON
6-2
BET & WIN 5 0
CYLINDER HEAD
REMOVAL
Remove the rear carrier.
Remove the frame body cover. (2-2)
Drain the coolant.
Disconnect the thermosensor wire from the
thermosensor.
Disconnect the water hose from the
thermostat housing.
Remove the spark plug cap.
Remove the two joint lock nuts on the front
of the exhaust muffler and then remove the
two exhaust muffler lock bolts.
The installation sequence is the reverse of
removal.
Remove the spark plug.
Remove the cylinder head bolts and the
cylinder head.
Remove the cylinder head gasket.
Cylinder head Bolts
Exhaust muffler joint lock nut
Bolts
Cylinder Head
Spark Plug
Spark Plug Cap
When installing the exhaust muffler,
first tighten the two nuts on the front
and then tighten the two bolts.
°Ø
Loosen the bolts diagonally in 2 or 3
times.
°Ø
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6. CYLINDER HEAD/CYLINDER/PISTON
6-3
BET & WIN 5 0
COMBUSTION CHAMBER
DECABONIZING
Remove the carbon deposits from the
combustion chamber
CYLINDER HEAD INSPECTION
Check the cylinder head for warpage with a
straight edge and feeler gauge.
Service Limit: 0.10mm replace if over
CYLINDER HEAD INSTALLATION
Install the cylinder head on the cylinder
properly.
Install a new cylinder head gasket onto the
cylinder.
Avoid damaging the combustion
chamber wall and cylinder mating
surface.
°Ø
Cylinder head Gasket
Combustion Chamber
Mating Surface
Be careful not to damage the mating
surfaces.
°Ø
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6. CYLINDER HEAD/CYLINDER/PISTON
6-4
BET & WIN 5 0
Cylinder Head Bolts Installation
Install and tighten the cylinder head bolts
diagonally in 2 or 3 times.
Torque: 14.7_ 16.66N-m
Install the spark plug.
Torque: 10.78_ 16.66N-m
CYLINDER/PISTON
CYLINDER REMOVAL
Remove the met-in box and seat.
Remove the frame body cover.
Remove the cylinder head. (6-3)
Remove the two exhaust muffler joint lock
nuts and two exhaust muffler lock bolts.
Remove the exhaust muffler.
Remove the cylinder.
Remove the cylinder gasket.
PISTON REMOVAL
Remove the piston pin clip to remove the
piston pin and piston.
Cylinder head Bolts
Piston Pin Clip
Exhaust Muffler Lock
Cylinder Head
Joint Lock Nuts
Piston
Spark Plug
Piston Pin
Do not pry between the cylinder and
crankcase or strike the fins.
°Ø
• Do not damage or scratch the piston.
• Do not apply side force to the
connect-ing rod when removing the
piston pin.
• Place clean shop towels in the crank-
case to keep the piston pin clip from
falling into the crankcase.
°Ø
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6. CYLINDER HEAD/CYLINDER/PISTON
6-5
BET & WIN 5 0
Spread each piston ring and remove by
lifting it up at a point just opposite the gap.
Remove the expander.
CYLINDER/PISTON INSPECTION
Check the cylinder and piston for wear or
damage.
Clean carbon deposits from the exhaust
port area.
Measure the cylinder bore at three levels of
A, B and C in both X and Y directions.
Avoid the port area. Take the maximum
figure measured to determine the cylinder
bore.
Service Limit: 39.05mm replace if over
Be careful not to damage the cylinder
inside wall.
°Ø
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6. CYLINDER HEAD/CYLINDER/PISTON
6-6
BET & WIN 5 0
Inspect the top of the cylinder for warpage.
Service Limit: 0.10mm replace if over
Measure the piston O.D. at a point 5mm
from the bottom of the piston skirt.
Service Limit: 38.90mm replace if below
Measure the piston-to-cylinder clearance.
Service Limit: 0.10mm replace if over
A Mark
The cylinder has an A mark or no mark
on it. When replacing the cylinder with
a new one, use a cylinder having the
same mark as the old one.
°Ø
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6. CYLINDER HEAD/CYLINDER/PISTON
6-7
BET & WIN 5 0
Measure the piston pin hole I.D.
Service Limit: 12.03mm replace if over
Measure the piston pin O.D.
Service Limit: 11.98mm replace if below
Measure the piston-to-piston pin clearance.
Service Limit: 0.03mm replace if over
PISTON RING INSPECTION
Measure each piston ring end gap.
Service Limits: Top/Second
0.40mm replace if over
CONNECTING ROD SMALL END
INSPECTION
Install the piston pin and bearing in the
connecting rod small end and check for
excessive play.
Measure the connecting road small end I.D.
Service Limit: 17.03mm replace if over
Piston Ring
Feeler Gauge
Set each piston ring squarely into the
cylinder using the piston and measure
the end gap.
°Ø
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6. CYLINDER HEAD/CYLINDER/PISTON
6-8
BET & WIN 5 0
PISTON/CYLINDER INSTALLATION
First install the expander in the second ring
groove.
Then install the top and second rings in
their respective ring grooves.
The piston rings should be pressed into the
grooves with even force.
After installation, check and make sure that
each ring is flush with the piston at several
points around the ring.
A ring that will not compress means that
the ring groove has carbon deposits in it and
should be cleaned.
Install a new cylinder gasket on the mating
surface between the cylinder and crankcase.
Make sure that the ring end gaps are aligned
with the piston ring pins in the ring
grooves.
Lubricate the cylinder inside and piston
rings with engine oil and install the piston
into the cylinder while compressing the
piston rings.
Install the cylinder head.
Torque: 14.7_ 16.66N-m
Install the exhaust muffler and tighten the
exhaust muffler joint lock nuts.
Torque: 9.8_ 13.72N-m
Tighten the exhaust muffler lock bolts.
Torque: 29.4_ 35.28N-m
Install the frame covers.
Expander
Top Ring (1st Ring)
Cylinder Gasket
Piston
Ring Pins
Second Ring
Be careful not to damage the piston.
°Ø
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6. CYLINDER HEAD/CYLINDER/PISTON
6-9
BET & WIN 5 0
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