Vespa smallframe engine strip down and rebuild.
12. Installing a Polini 130cc cylinder kit.
Now we are getting to the exciting bit - power!
This page should be read if you are not installing a Polini kit,
as many of the principles are excactly the same whether you are
installing the stock cylinder on your 50 special or installing a
full house Malossi 133 kit (and some are very different. If you're
installing a Zirri MR2000 kit I hope you are good mates with the
guy who owns a great machine shop)
The first thing you need to do is make sure your cylinder fits.
Exactly. You don't want a gap of even 1mm if you can avoid it. If
your cases are good then you shouldn't have a problem I found the
quality of the Polini 130 cylinder excellent., and it was a perfect
fit without any filing needed to my ET3 cases. File any rough burrs
smooth, keeping the debris out of the cylinder or engine.
If you are just fitting the cylinder straight on without any porting
then you won't generate vastly more power than a stock ET3, but
it lasts longer and you will get a nice fat tourque curve to match.
This is exactly what I did. porting and big carbs would ne nice
but I can only afford to do things to the smallframes.com project
bike a little bit at a time, and a full tuning job is beyond my
price range and budget. There would be no job at all if my old cylinder
was still up to the job of hauling my fat arse across London. This
approach also saves me the temptation of using all that new power
and seizing the engine.
What is it about the Polini 130cc kit that makes it more powerful
than a 125 ET3 Cylinder anyway? It's only a teaspoon more in displacement
terms after all. Let's take a look:
Ignoring the obviously not-well condition of the ET3 cylinder these
cylinders look very similar. The main difference visible here is
the difference in the case port windows. On the ET3 they cylinder
liner section is complete around the base, with the port windows
cut into the sides, whereas those on the Polini cylinder are larger
and start at the base of the cylinder liner. You can see this better
at the side. What is not readily apparent from a visual comparison,
but becomes clear when you mate the Polini cylinder to the cases
is that the size of the port windows is much larger on the cylinder
than on a standard set of cases.
To get the full power from the kit the case should be opened up
by grinding and filing material away until it matches the ports
on the cylinder. This will make a vast difference to the power,
but if you don't want to do this for whatever reason you will still
get a useful power boost from the transfer ports further up the
As you can see from the above shot the 3-transfer ET3 cylinder
has nothing on the 7-transfer Polini cylinder! To be honest I don't
know why any of this lot makes it better than the ET3, but clearly
Polini do, so we don't need to worry! Even on an otherwise bog-standard
ET3 engine (but with a Leovinci pipe) the Polini cylinder means
noticeably more tourque and tractability right through the rev range
and it doesn't run out of puff the way an ET3 will, giving you over
5hp between 6500 and 9500 rpm
A comparison of the cylinder heads seems to show that they are
very similar, but a visual inspection is not enough to say one way
or the other. I think the Polini head has a larger combustion chamber.
The pistons on the other hand do look different. The
Polini one has a larger intake window and the side skirt is cut
away more than the ET3 one.
So that's that. Now to go about putting all this in
our engine. If you arrived here after all the other engine articles
then you have a built up engine minus a cylinder, piston and head,
(and clutch and flywheel, but that's not relevant until the next
page). If you came straight here and your engine is still complete
then remove your cylinder and piston first.
The following instructions assume you are replacing a 125cc cylinder
with a 130cc kit. If you have a smaller displacement engine you
need to consider whether other procedures are necessary - for example
changing the crank, fitting long cylinder studs to your engine,
etc. These instructions also do not go into the mechanics of porting
the cases to match the cylinders, or installing a cut crank. (The
Polini instructions give you advice on how to cut your own crank.
Don't do it! buy a Mazzuchelli once instead!)
Before starting, wash your cylinder and piston with
petrol, and then give the inside of the cylinder a smearing of 2-stroke
First you need to install the piston. Put the caged
roller bearings in the conrod small end, and hold the piston over
it with the intake hole facing the intake (and the arrow on the
top facing the exahust), lining up the holes, then pass the
wristpin through the piston, conrod and other side of the piston.
Secure it in place with the wire clips provided (circlips on a stock
On the piston there are two grooves
for piston rings, Look closely at them and you'll notice there
are two "stops" in the grooves. These are where you
locate the ends of the piston rings. Fit the piston rings loosely
now and roughly in the right
place. The idea is the ring gaps will not pass over any of the
ports, and the cylinder will keep them compressed.
Smear a thin coat of gasket sealer or silicone batroom
sealer on both sides of a new cylinder base gasket. Slide the new
cylinder (minus head!) down the cylinder studs, taking care not
to drop it, damage the piston or do anything bad at all. Put the
piston into the opening of the cylinder, whilst at the same time
pinching the top ring to compress it with the ring gap exactly
on the "stop". When this is done repeat the process
for the second piston ring. Once they are both in the cylinder it
should be possible to slide the piston into the cylinder and the
cylinder down onto the cases. If it's still possible to turn the
engine then you have done it right and can put the cylinder head
on. Smear a thin coating of gasket sealer onto the mating surface
of the cylinder head and slide it down the studs into place, with
the spark plug hole facing the flywheel side of the engine.
Now replace the washers and nuts on these studs and
torque them down equally, going round the head.