Yamaha MTN850-A 2017, Service Manual, Page: 299 / 560

CONNECTING RODS AND PISTONS
5-64
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.
EAS30749
CHECKING THE PISTON PIN
The following procedure applies to all of the pis-
ton pins.
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 inside 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.
EAS30750
CHECKING THE CONNECTING RODS
1. Measure:
• Crankshaft-pin-to-big-end-bearing clearance
Out of specification

Replace the big end
bearings.
Top ring
End gap (installed)
0.15–0.25 mm (0.0059–0.0098 in)
End gap limit
0.50 mm (0.0197 in)
2nd ring
End gap (installed)
0.30–0.45 mm (0.0118–0.0177 in)
End gap limit
0.80 mm (0.0315 in)
b. Upper of cylinder
Piston pin outside diameter
16.990–16.995 mm (0.6689–
0.6691 in)
Limit
16.970 mm (0.6681 in)
a
b
Piston pin bore inside diameter
17.002–17.013 mm (0.6694–
0.6698 in)
Limit
17.043 mm (0.6710 in)
Piston-pin-to-piston-pin-bore clearance = Piston
pin bore inside diameter “b” - Piston pin outside
diameter “a”
Piston-pin-to-piston-pin-bore
clearance
0.007–0.023 mm (0.0003–0.0009
in)
Oil clearance
0.027–0.051 mm (0.0011–0.0020
in)