Yamaha YZ125 2022, Owner'S Service Manual, Page: 169 / 278

CYLINDER HEAD, CYLINDER AND PISTON
5-9
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.
a. Place a straightedge “1” and a thickness
gauge “2” across the cylinder head.
b. Measure the warpage.
c. If the limit is exceeded, resurface the cylin-
der head as follows.
d. Place a 400–600 grit wet sandpaper on
the surface plate and resurface the cylin-
der head using a figure-eight sanding pat-
tern.
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-
inder bore gauge.
TIP
Measure the cylinder bore “C” by taking side-to-
side and front-to-back measurements of the cyl-
inder.
b. If out of specification, replace the cylinder,
and replace the piston and piston rings as
a set.
c. Measure the piston skirt diameter with the
micrometer.
d. Measure the specific distance “a” from the
bottom edge of the piston.
Warpage limit
0.03 mm (0.0012 in)
G088999
1
1
2
Bore
54.000–54.014 mm (2.1260–
2.1265 in)
Wear limit
54.064 mm (2.1285 in)
“C” = maximum of D
1
, D
2
, D
3
, D
4
, D
5
, D
6
G089000