Checking The Piston Ring; Checking The Piston Pin - Yamaha YZ125X 2021 Owner's Service Manual

EAM30548

CHECKING THE PISTON RING

1. Measure:
• Piston ring side clearance
Out of specification  Replace the piston
and piston ring as a set.
TIP
Before measuring the piston ring side clearance,
eliminate any carbon deposits from the piston
ring grooves and piston ring.
Top ring
Ring side clearance
0.035–0.070 mm (0.0014–0.0028
in)
Side clearance limit
0.100 mm (0.0039 in)
G089002
2. Install:
• Piston ring
(into the cylinder)
TIP
Use the piston crown to level the piston ring near
bottom of cylinder "a", where cylinder wear is
lowest.
3. Measure:
• Piston ring end gap
Out of specification  Replace the piston
ring.
End gap limit
1.20 mm (0.0472 in)
G089003
b. Upper of cylinder
CYLINDER HEAD, CYLINDER AND PISTON
EAM30549

CHECKING THE PISTON PIN

1. Check:
• Piston pin
Blue discoloration/grooves  Replace the
piston pin, and then check the fuel and en-
gine mixing oil.
2. Measure:
• Piston pin outside diameter "a"
Out of specification  Replace the piston pin.
G089004
3. Measure:
• Piston pin bore inside diameter "b"
Out of specification  Replace the piston.
G089005
4. Calculate:
• Piston-pin-to-piston-pin-bore clearance
Out of specification  Replace the piston pin
a and piston as a set.
Piston-pin-to-piston-pin-bore clearance =
Piston pin bore inside diameter "b" -
Piston pin outside diameter "a"
b
5-10
Piston pin outside diameter limit
14.975 mm (0.5896 in)
Piston pin bore inside diameter
limit
15.040 mm (0.5921 in)
b