Yamaha YZ125 2009, Owner'S Service Manual

Page: 103 / 234

4-20
CYLINDER HEAD, CYLINDER AND PISTON
3. Measure:
• Cylinder head warpage
Out of specification
→
Resurface.
Warpage measurement and resurfacing
steps:
a. Attach a straightedge "1" and a thickness
gauge "2" on the cylinder head.
b. Measure the warpage.
c. If the warpage is out of specification, resur-
face the cylinder head.
d. Place a 400–600 grit wet sandpaper "3" on
the surface plate, and resurface the head "4"
using a figure-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 scratch-
ing the aluminum.
2. Inspect:
• Cylinder inner surface
Score marks
→
Repair or replace.
Use #400–600 grit wet sandpaper.
Do not rebore the cylinder.
3. Measure:
• Cylinder bore "C"
Use cylinder gauge "1".
Out of limit
→
Replace.
Measure the cylinder bore "C" in parallel (A, B,
C) to and at right angles to the crankshaft (a, b).
Then, find the average of the measurements.
Cylinder head warpage:
Less than 0.03 mm (0.0012 in)
Standard Wear limit
Cylinder
bore "C"
54.000–54.014
mm (2.1260–
2.1265 in)
54.1 mm
(2.130 in)
Taper "T" —
0.05 mm
(0.0020 in)
"C" = Maximum Aa–Cb
"T" = (Maximum Aa, or Ab) - (Maximum Ba,
or Bb)