Vespa smallframe engine strip down and rebuild.
4. Removing the flywheel and stator.
The flywheel on a vespa serves four purposes. Firstly like any
flywheel it keeps the engine turning on the idle strokes, secondly
it directs air to cool the cylinder, and thirdly, in conjunction
with the stator plate, it generates electricity to power the lights
and horn and spark. Finally it also provides the ignition timing.
So obviously you want to be quite careful when removing it.
To actually remove it you need to remove the flywheel cover if
you have not already done so, which will allow you to unscrew the
nut in the centre. You will almost certainly need to prevent the
flywheel from moving when you do this. You can use the Vespa flywheel
holding tool or you can make something similar. Basically you need
a fairly strong piece of metal that screws into one or two of the
holes vacated by the flywheel cover and jams against the fins on
the flywheel, preventing it from turning.
On some early smallframes there is a circlip which should be left
in place. As you undo the nut it pushes against the circlip and
forces the flywheel up the shaft. On most others you will need to
use the Vespa smallframe flywheel puller. This screws into the threads
in the centre of the flywheel and you tighten a bolt in the centre
to put pressure on the shaft. A few taps with a hammer on this bolt
may well be necessary to loosen the flywheel so that you can remove
There is quite a strong magnetic bond to the stator (which is used
to generate electricity) so be careful removing the flywheel. When
you remove it make sure that the woodruff key that locates it is
not lost. You'll notice that the flywheel has six magnets around
the inside separated by grooves. These pass over the various coils
and generate the electricity.
If you look at the stator you should notice a raised mark cast
into the stator plate that (on a stock motor) lines up with a similar
mark cast into the engine case. This is the timing mark and the
stator will be lined up with this at the factory to save them the
bother of timing the engine properly when churning them out by the
thousand. It's close enough, but you may find that the marks don't
quite line up in which case the engine may well have been timed
accurately later. You may also find that the marks don't line up
at all. This may mean that the engine is timed incorrectly, but
may well also mean that the original ignition timing is no longer
appropriate - usually because the engine is in some way no longer
stock. Changes of wiring harness and stator, cylinder kits, cylinders
from other models etc could all mean timing changes or at least
that the marks won't line up.
On the ET3 engine pictured here you can see the electronic ignition
contact. When this lines up with a point on the inside of they flywheel
where the magnets either side of one of the grooves overlap each
other. When this point passes the contact a signal is sent to the
CDI box which then sends the spark voltage along the HT lead. You
can see a white line on the top of the contact. If you look at the
flywheel there is a hole with a rubber bung in. Remove this bung
to reveal two small marks in the flywheel. When these marks line
up with the white line on the contact (viewable through the hole)
the spark plug will fire. On a points ignition model there is a
pad that bumps against the points to send the spark.
After noting where the stator lines up you can loosen the three
bolts that hold it in place. Note that there is a slot which allows
the stator to be rotated about ten degrees in either direction to
adjust the timing. Remove the bolts to remove the stator plate,
carefully feeding the wires through from the outside of the engine.
Now you can see the flywheel-side oil seal. This can be removed
and replaced with a new one without needing to remove the engine
from the bike. Of course if you're taking the engine apart you should
replace it as a matter of course. It is removed from the outside
of the engine.