Yamaha yz250 2015, Owner'S Manual

Page: 79 / 174

4-15
CYLINDER HEAD, CYLINDER AND PISTON
REMOVING THE PUSH ROD
1. Remove:
• Bolt (push rod) "1"
• Push rod "2"
Insert the set pin "3" included in own-
er's tool kit to remove the bolt (push
rod).
Be sure to use the set pin. If the set
pin is not used, the power valve
constituent parts will result in
damage.
REMOVING THE PISTON AND
PISTON RING
1. Remove:
• Piston pin clip "1"
Before removing the piston pin clip,
cover the crankcase with a clean rag
to prevent the piston pin clip from fall-
ing into the crankcase cavity.
2. Remove:
• Piston pin "1"
• Piston "2"
• Small end bearing "3"
Before removing each piston pin, de-
burr the clip groove and pin hole area.
If the piston pin groove is deburred
and the piston pin is still difficult to re-
move, use the piston pin puller set
"4".
Do not use a hammer to drive the
piston pin out.
3. Remove:
• Piston ring "1"
Take care not to scratch the piston or
damage the piston ring by expanding
it more than necessary.
CHECKING THE CYLINDER HEAD
1. Eliminate:
• Carbon deposits
Use a rounded scraper "1".
Take care to avoid damaging the
spark plug threads. Do not use a
sharp instrument. Avoid scratching
the aluminum.
2. Inspect:
• Cylinder head water jacket
Crust of minerals/Rust
→
Re-
place.
3. Measure:
• Cylinder head warpage
Out of specification
→
Resurface.
Warpage measurement and resur-
facing steps:
a. Attach a straightedge "1" and a
thickness gauge "2" on the cylin-
der head.
b. Measure the warpage.
c. If the warpage is out of specifica-
tion, resurface the cylinder head.
d. Place a 400–600 grit wet sandpa-
per "3" on the surface plate, and
resurface the head "4" using a fig-
ure-eight sanding pattern.
To ensure an even surface rotate the
cylinder head several times.
CHECKING THE CYLINDER
1. Eliminate:
• Carbon deposits
Use a rounded scraper "1".
Do not use a sharp instrument. Avoid
scratching the aluminum.
2. Inspect:
• Cylinder inner surface
Score marks
→
Repair or replace.
Use #400–600 grit wet sandpa-
per.
Do not rebore the cylinder.
Piston pin puller set:
YU-01304/90890-01304
Cylinder head warpage:
Less than 0.03 mm
(0.0012 in)