VESPA 633844 IT, Service Manual

Page: 128 / 308

Inspecting the piston
- Measure the diameter of the wrist pin seat on the
piston.
- Calculate the piston pin coupling clearance.
- Measure the outside diameter of the piston, per-
pendicular to the gudgeon pin axis.
- Take the measurement at 5 mm from the base in
the position shown in the figure.
- Carefully clean the seal housings.
- Measure the coupling clearance between the
seal rings and the grooves using suitable sensors,
as shown in the diagram.
- If the clearance is greater than that indicated in
the table, replace the piston.
N.B.
MEASURE THE CLEARANCE BY INSERTING
THE BLADE OF THE FEELER GAUGE FROM
THE SECOND SEAL SIDE.
N.B.
THE PIN HOUSINGS HAVE 2 LUBRICATION
CHANNELS. FOR THIS REASON MEASURE-
MENT OF THE DIAMETER MUST BE CARRIED
OUT ACCORDING TO THE AXIS OF THE PIS-
TON.
Characteristic
Wrist pin seat on the piston: Standard diameter
15.001 ÷ 15.006 mm
Diameter of the wrist pin seat on the piston:
Standard clearance
0.001 ÷ 0.010 mm
piston diameter
71.953 - 71.981 mm
Fitting clearance
Top piston ring - standard coupling clearance
0.015 - 0.06 mm Top piston ring - maximum
clearance allowed after use 0.07 mm Middle
piston ring - standard coupling clearance 0.015
- 0.06 mm Middle piston ring - maximum clear-
ance allowed after use 0.07 mm oil scraper ring
- standard coupling clearance 0.015 - 0.06 mm
oil scraper ring - maximum clearance allowed
after use
0.07 mm
Engine
Vespa GTV 250 i.e.
ENG - 34