Yamaha YZ250 2022, Owner'S Service Manual

Page: 169 / 274

CYLINDER HEAD, CYLINDER AND PISTON
5-7
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 “b” from the
stepped surface “a” on inside of the piston.
Warpage limit
0.03 mm (0.0012 in)
G088999
1
1
2
Bore
66.400–66.414 mm (2.6142–
2.6147 in)
Wear limit
66.464 mm (2.6167 in)
“C” = maximum of D
1
, D
2
, D
3
, D
4
, D
5
, D
6
G089000