VESPA LX 4T USA Workshop Manual page 89

LX 4T USA
- Carry out the measurement at 27 from the piston crown as shown in the figure.
Inspecting the cylinder
- Using a bore meter, measure the inner cylinder
diameter at three different points according to the
directions shown in the figure.
- Check that the coupling surface with the head is
not worn or misshapen.
- Pistons and cylinders are classified into catego-
ries based on their diameter. The coupling is car-
ried out in pairs (A-A, B-B, C-C, D-D).
Characteristic
Maximum allowable run-out:
0.05 mm
- The cylinder rectifying operation should be car-
ried out with a surfacing that respects the original
angle. at 120° crossed.
- The cylinder surface roughness should be of
R.A.= 0.30 ÷ 0.50.
- This is indispensable for a good seating of the
sealing rings, which in turn minimises oil consump-
tion and guarantees optimum performance.
- The pistons are oversized due to cylinder rectifi-
cation and are subdivided into two categories 1st
and 2nd with 0.2-0.4mm oversize. They are also
classified into 4 categories A-A, B-B, C-C, D-D.
Inspecting the piston rings
- Alternately insert the three sealing rings into the cylinder, in the area where it retains its original di-
ameter. Using the piston, insert the rings perpendicularly to the cylinder axis.
- Measure the opening, see figure, of the sealing rings using a thickness gauge.
- If any measurements are greater than specified, replace the piston rings.
N.B.
BEFORE REPLACING ONLY THE PISTON RINGS, ENSURE THAT THE CLEARANCE BETWEEN
THE PISTON RINGS AND THE PISTON RING GROOVES, AND BETWEEN THE PISTON AND THE
CYLINDER, IS AS SPECIFIED. IN ANY CASE, NEW PISTON RINGS USED IN COMBINATION WITH
A USED CYLINDER MAY HAVE DIFFERENT BEDDING CONDITIONS THAN THE STANDARD.
Engine
ENG - 89