Checking The Cylinder Head; Checking The Cylinder And Piston - Yamaha YZ125 2022 Owner's Service Manual

G088999
EAM30545

CHECKING THE CYLINDER HEAD

1. Eliminate:
• Combustion chamber carbon deposits (with a
rounded scraper "1")
TIP
Do not use a sharp instrument to avoid damag-
ing or scratching:
• Spark plug bore threads
2. Check:
• Cylinder head
Damage/scratches  Replace.
• Cylinder head water jacket
Mineral deposits/rust  Eliminate.
3. Measure:
• Cylinder head warpage
Out of specification  Resurface the cylinder
head.
Warpage limit
0.03 mm (0.0012 in)
a. Place a straightedge "1" and a thickness
gauge "2" across the cylinder head.
CYLINDER HEAD, CYLINDER AND PISTON
b. Measure the warpage.
c. If the limit is exceeded, resurface the cylin-
d. Place a 400–600 grit wet sandpaper on
TIP
To ensure an even surface, rotate the cylinder
head several times.
EAM30546

CHECKING THE CYLINDER AND PISTON

1. Check:
• Piston wall
• 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 cyl-
TIP
Measure the cylinder bore "C" by taking side-to-
side and front-to-back measurements of the cyl-
1
inder.
"C" = maximum of D
G089000
b. If out of specification, replace the cylinder,
1
c. Measure the piston skirt diameter with the
2
d. Measure the specific distance "a" from the
5-9
der head as follows.
the surface plate and resurface the cylin-
der head using a figure-eight sanding pat-
tern.
inder bore gauge.
Bore
54.000–54.014 mm (2.1260–
2.1265 in)
Wear limit
54.064 mm (2.1285 in)
, D , D , D
1 2 3
and replace the piston and piston rings as
a set.
micrometer.
bottom edge of the piston.
, D , D
4 5 6