Yamaha YZ125 2019 Owner's Service Manual page 107

3. Measure:
• Cylinder head warpage
Out of specification → Resurface.
Cylinder head warpage:
Less than 0.03 mm (0.0012 in)
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.
CYLINDER HEAD, CYLINDER AND PISTON
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 specification → 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
bore "C"
Taper "T"
"C" = Maximum Aa–Cb
"T" = (Maximum Aa, or Ab) - (Maximum Ba,
or Bb)
4-20
Standard Wear limit
54.000–54.014
54.100 mm
mm
(2.1299 in)
(2.1260–2.1265 in)
0.050 mm
—
(0.0020 in)