Yamaha YZ125(D), Owner'S Service Manual

Page: 76 / 168

4-14
CYLINDER HEAD, CYLINDER AND PISTON
REMOVING THE PUSH ROD
1. Remove:
• Bolt (push rod) "1"
• Push rod "2"
Set the collar "3" included in owner's
tool kit to remove the bolt (push rod).
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.
3. Measure:
• Cylinder bore "C"
Use cylinder gauge "1".
Out of specification
→
Replace.
Measure the cylinder bore "C" in par-
allel (A, B, C) to and at right angles to
the crankshaft (a, b). Then, find the
average of the measurements.
Piston pin puller set:
YU-1304/90890-01304
Cylinder head warpage:
Less than 0.03 mm
(0.0012 in)