Betamotor EVO 4t, Руководство по ремонту, Страница: 62 / 80

56
ASSEMBLY
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7
mm
4.14 PISTON AND CYLINDER
4.14.1 measurements and checks
4.14.1.1 Cylinder
Check whether the lower and upper surfaces of the
cylinder show deformation. Use a surface plate and
a dial gauge to measure for flatness.
The limit value for deformation is 0.05mm.
Recondition the cylinder in the event of out-of-limits
values.
Check whether the inner surface of the cylinder
shows signs of seizure or excessive wear. If it does,
recondition the surface.
Measure the internal diameter of the cylinder on
two axes at right angles to each other, X (parallel
to the crankshaft) and Y (perpendicular to the cran-
kshaft) and at three different levels. Use the largest
value measured for subsequently determining the
maximum piston-cylinder clearance.
N.B. The front side of the cylinder (exhaust side) is
marked with the letter for the selection grade. This is
needed for correct matching with the piston, as both
must be of the same grade (the piston is marked on
the crown with its diameter measurement).
SelecTion gRade
A 76,960±0,005mm
B 76,970±0,005mm
C 76,980±0,005mm
4.14.1.2 Piston
Check the sliding surfaces for wear and scoring, and
look for dark areas below the oil scraper ring groo-
ve. Replace the piston if these are found.
Use a micrometer to measure the diameter of the
piston in a direction perpendicular to the piston pin,
at a distance of 7mm from the base of the skirt.
Once the measurement of the diameters of the
cylinder and piston has been carried out, the clea-
rance between the two can be determined:
limit of play 0.1mm
Measure the thickness of the two compression
rings.
Minimum thickness of upper ring: 0.87 mm
Minimum thickness of lower ring: 0,77 mm
Change the compression rings if their thicknesses
are lower than these limits.