Yamaha UMAX, Service Manual

Page: 170 / 320

CRANKSHAFT AND PISTON
5-31
3. Measure:
• Piston ring end gap
Out of specification
→
Replace the piston
ring.
TIP
The oil ring expander spacer’s end gap cannot
be measured. If the oil ring rail’s gap is exces-
sive, replace all three piston rings.
CHECKING THE PISTON PIN
1. Check:
• Piston pin
Blue discoloration/grooves
→
Replace the
piston pin and then check the lubrication sys-
tem.
2. Measure:
• Piston pin outside diameter “a”
Out of specification
→
Replace the piston pin.
3. Measure:
• Piston pin bore diameter “b”
Out of specification
→
Replace the piston.
4. Calculate:
• Piston-pin-to-piston-pin-bore clearance
Out of specification
→
Replace the piston pin
and piston as a set.
CHECKING THE CRANKSHAFT AND
CONNECTING ROD
1. Measure:
• Crankshaft runout
Out of specification
→
Replace the crank-
shaft.
2. Check:
• Crankshaft journal surfaces
• Crankshaft pin surfaces
Scratches/wear
→
Replace the crankshaft.
• Crankshaft sprocket
Damage/wear
→
Replace the crankshaft.
3. Measure:
• Crankshaft pin outside diameter “a”
Out of specification
→
Replace the crank-
shaft.
Piston ring
Top ring
End gap limit
0.60 mm (0.0236 in)
2nd ring
End gap limit
0.80 mm (0.0315 in)
Piston pin outside diameter
19.995–20.000 mm
(0.7872–0.7874 in)
Limit
19.975 mm (0.7864 in)
Piston pin bore inside diameter
20.004–20.015 mm
(0.7876–0.7880 in)
Limit
20.045 mm (0.7892 in)
• Piston-pin-to-piston-pin-bore clearance =
Piston pin bore diameter “b” -
Piston pin outside diameter “a”
Piston-pin-to-piston-pin-bore
clearance
0.004–0.020 mm
(0.0002–0.0008 in)
Runout limit
0.05 mm (0.002 in)