It was back in November 2025 that Polini set out their intentions to create a fresh set of casings for the PX. This was welcome news as with almost a decade passing since the last 125/150 (small-block) machines left the showrooms and a quarter of a century since the 200 (large-block) officially ceased production, finding a used set that hasn’t been welded, sheared, ported and otherwise faffed about with is getting harder every week. As for new old stock, we can occasionally source them but be prepared to pay well into four figures for the privilege.
Now, thanks to Polini, we’re able to offer you both small block, and large block casings off the shelf for around £800. Even better, these aren’t just a knock off version of 50 year old Piaggio engineering, they’re almost an entire redesign. Whilst the original casings did a good job of containing all the parts of a 9bhp engine designed to pop down the shops, once performance kits came into the equation their weaknesses began to show. What the engineers at Polini have done is take the basic format and re-engineer it to suit the 21st century.
Some basics
Before getting into the detail, it’s worth clarifying some basic points. Firstly, the casings are available in both large and small-block variants, so you can still fit your barrel of choice. With the exception of the small block’s engine mounts, which are now changed to the far superior large block type, both casings have the same specification.
A larger ignition side bearing (25x52x15) has been incorporated for better smoothness and durability, whilst the M7 through bolts have been replaced with M8 hexagonal screws. Other than that it’s possible to mount every component from a standard PX should you so wish. Yes, that includes electric start and autolube.
The cases are manufactured in Italy from light aluminium alloy (Al Si10 Mg T6 for the metallurgists out there), are cast in shell moulds, and treated thermally to ensure maximum mechanical strength, thermal stability and durability. During production the two halves are matched and numbered, ensuring that machining tolerances are maximised. This is critical as there central gasket has been eliminated. Although the centre gasket maintains good insulation properties, it doesn’t allow for perfect alignment or manufacturing tolerances. So the best option for achieving perfect alignment is to omit the centre gasket and use silicone or sealant paste instead. The engineers at Polini are also keen to point out that the machining required to locate the crank bearings is also carried out once the casings have been paired. In short, every effort is made to ensure correct alignment, in fact this even extends to the exhaust mounting point, which is now machined perfectly flat to enable easier fitting.
There’s also extra metal at strategic points in the casing, notably strengthening webs around the gearbox and kickstart shaft, plenty of meat to allow for porting around the crankcase mouth and even material in the swing arm that can be machined away without losing any structural integrity should a wider tyre be your thing.
Another weakness of the PX design is the gearbox oil capacity. As standard this is a paltry 250ml, which leaves high performance engines somewhat lacking in the lubrication department. By moving the filler hole further up the casing this capacity is almost doubled, and a clever modification to the optional clutch cover eliminates blow out from the breather hole, but more of that in a moment.
Finally in our quick overview, the Polini logo sits on the flat where the engine number is usually stamped. For jurisdictions that don’t require an engine number it’s a subtle indication of what lies beneath. However, the logo isn’t impressed into the casing it stands proud, so where an engine number is required it can be removed with relative ease and a number of the owner’s choice stamped in place.
Having set out the basics, it’s already clear that these are an advanced product, but there’s more. In fact, although standard internals can be used, the Polini casings form part of an integrated system which includes carburetion, crank, clutch and gearbox. For now, we'll concentrate on the casings.
Breathe in
Having explored the weaknesses of the standard casing and looked at common tuning upgrades, the Polini engineers have devised a solution that enables better sealing of the rotary valve with a standard carb. This involves fitting a plate that contains a flat reed valve between the casing and air box. It’s a clever system that requires no frame modifications but dramatically increases induction efficiency. If autolube is required, it can be fitted as normal or alternatively a blanking plate can be supplied to seal off the autolube system.
For users who wish to fit a larger, non-SI carb and manifold there’s no need to purchase a separate casing. A reed valve pack, or box, is available that allows the fitting of a true reed valve and manifold. This makes the casings extremely flexible in terms of carburettor options although, obviously, fitting a non SI carb will remove the option of autolube.
Another modification to the crankcase is that the clutch side bearing can be removed from the outside of the engine. Theoretically, this can be done without splitting the casings, but ask anyone that’s worked on a KTM how much fun that can be. The main aim here is to allow the oil seal to be fitted from the crankcase side, reducing the chances of it being ‘sucked’ into the crank web. So, for 99% of users, it’ll still be necessary to split the casings in order to replace the oilseal and bearing, but the chances of needing to do the job in the first place are much reduced.
Breathe out
With the crankcases offering twice the original oil capacity, it’s natural for any experienced rider to be concerned about oil blowing out of the breather hole. This has been a problem since Vespa’s early days, but one that’s finally been solved by as part of Polini’s new clutch cover’s design.
Amongst the many improvements offered by this casting are a series of cisterns in the casing. These have been CAD modelled to allow air flow whilst ensuring oil remains where it’s meant to be.
The cover also has the necessary clearance to allow the fitting of performance clutches without the need to remove internal webs and is machined to allow the fitment of a wider tyre. It also, thankfully, has a Lambretta type ‘claw’ that holds the cable nipple in place, allowing the clutch to be easily disconnected for maintenance. This new arm is also secured to the operating rod on splines, enabling even more adjustment to suit any given riding style. Best of all, this cover can also be fitted to a set of standard casings and we’re already selling them in large numbers for just that purpose.
It’s not often we’ll claim that a product sets an entirely fresh standard for Vespa enthusiasts, but the Polini casings are worthy of that accolade.
For more information on the Polini range give our sales team a call on: 01226 203377 or drop them a line at: sales@rondaleyscooters.co.uk
A more detailed article covering the Polini casings, clutch and gearbox will appear in a future edition of Scootering Magazine. Or you can follow this link to see Polini’s video of this outstanding product.
