Yamaha YZ 2018 Series Owner's Service Manual page 227

EASB29B230
REMOVING THE PISTON
1. Remove:
• Piston pin clip "1"
• Piston pin "2"
• Piston "3"
ECA
NOTICE
Do not use a hammer to drive the piston pin
out.
TIP
• Before removing the piston pin clip, cover the
crankcase opening with a cloth to prevent the
piston pin clip from falling into the crankcase.
• Before removing the piston pin, deburr the
piston pin clip's groove and the piston pin's
bore area. If the piston pin groove is deburred
and the piston pin is still difficult to remove,
use the piston pin puller set "4".
Piston pin puller set
90890-01304
Piston pin puller
YU-01304
1
2. Remove:
• Top ring
• Oil ring
TIP
When removing a piston ring, open the end gap
with your fingers and lift the opposite end gap
of the piston ring over the piston crown.
CYLINDER AND PISTON
EASB29B231
CHECKING THE CYLINDER AND PISTON
1. Check:
• Piston wall (Sidewall)
• Cylinder wall
Vertical scratches  Replace the cylinder,
and replace the piston and piston rings as a
set.
2. Measure:
• Piston-to-cylinder clearance
▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼
a. Measure the cylinder bore "C" with the cylin-
der bore gauge.
TIP
Measure cylinder bore "C" by taking side-to-
side and front-to-back measurements of the
cylinder.
Bore
77.000–77.010 mm (3.0315–
3.0319 in)
Wear limit
77.060 mm (3.0339 in)
"C" = maximum of D
b. If out of specification, rebore or replace the
cylinder, and replace the piston and the pis-
ton rings as a set.
c. Measure the piston outside diameter "a" at
the measuring point (from piston skirt bot-
tom) "b" with the micrometer.
5-30
, D , D , D , D , D
1 2 3 4 5 6