Chapter 8 – Cylinder and Piston
Service Manual AJP PR3/PR4 8 - 6
Note:
If the cylinder is rectified to assemble a
larger piston, make sure that the clearance
between the cylinder and the new piston is
within the specified limits, to ensure the
correct operation of the engine. Reduced
clearance may cause overheating of the
engine and seize (merge) the piston in the
cylinder. Excessive clearance may result in
excessive premature wear and loss of engine
power due to the low compression in the
cylinder.
With the help of a straight edge and feeler
gauge measure the warpage in the cylinder
surface.
Cylinder warpage limit:
0,05mm
With the help of an internal micrometer
measure the bore at three different points: at
the top, the center and the base of the piston
stroke. Measure horizontally and vertically (X
and Y axis) at 90º.
Service limit
125cm
3
56,60mm
200/240cm
3
69,10mm
Roundness
0,05mm
Piston / Rings
With the help of a micrometer measure the
outer diameter of the piston skirt, as shown in
the picture.
Place the micrometer at the height indicated
in the dimension "a" around 6 mm from the
base of the piston (ideal reference to inner
edge).
The clearance between the cylinder and the
piston is calculated as follows: Cylinder bore
subtracting the external diameter of the
piston. Always use the highest measurement
found in the three measured positions earlier
in cylinder measurement.
Service
limit
125cm
3
56,40mm
150cm
3
60,80mm
200/240cm
3
68,80mm
Clearance between
cylinder / piston
0,2mm
If the service limit of the piston diameter is
exceeded it must be replaced along with the
rings and can use the same cylinder if it
undamaged and its diameter is within the limit.
In the case of the cylinder is damaged or its
diameter is no longer in service limit, cylinder,
piston and rings must be replaced all together.
