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ENGINE CHECKS AND ASSEMBLY
3.12 THERMAL UNIT
Before assembling the thermal unit, thorough
checks must be carried out on the cylinder, pis-
ton, pin bolt, elastic bands described hereafter.
3.12.1 CHECKING THE PISTON
AND ELASTIC BANDS
Remove carbon sediment from the top of the pis-
ton and wash the entire piston with specific de-
greasing agent.
Do not use sharp points or tools for cleaning.
Then carry out a careful analysis of the piston
which must be free from forcing, scratches, cracks
or any type of damage.
Analyse the external diameter of the piston.
The diameter must be measured at a distance X
from the inner edge of the skirt, perpendicularly
to the axis of the pin bolt.
Refer to the following table for the piston's selec-
tion classes.
Position to measure the outside diameter of the
piston.
Displace-
ment [cm³]
Measure-
ment X [mm]
Bore [mm]
A B C
250 18 66.345÷66.354 66.355÷66.364 66.365÷66.375
300 18 72,945÷72,954 72,955÷72,955 72,965÷72,975
In order to check the condition of the elastic
bands, the band must be inserted in the cylinder
so that its axis is aligned with the axis of the cylin-
der. Then measure the distance between the two
opposite ends.
The elastic bands must be positioned at about 31
mm from the cylinder's upper edge.
The respective service limits are set out in the
table below.
First check of elastic bands.
Displacement
[cm³]
Service limit of elastic
band [mm]
250 - 300 0.4
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