Piston -POLINI 2023 CVF- Ø 68,5 mm - Vespa PX200, Rally200, Cosa200 (Polini 210/221)
POLINI has revised the piston design from 2023: The new POLINI 2023 CVF piston has a significantly more massive design and uses an overflow system through the piston skirt - similar in concept to familiar CVF solutions. The aim is a defined gas flow in the area of the transfer ports and an overall robust piston design for stable, everyday setups.
Info: POLINI aluminum cylinders until 2023 (other piston type)
POLINI aluminum cylinders were supplied with a different piston type up to and including 2023. The new 2023 CVF piston can easily replace this older piston and is often used as an update/replacement. As always, it is important to use the correct ring assembly (aluminum/cast iron) and the correct running clearance.
Important: What is new - and what does this mean for the selection?
New piston type (from 2023): stronger piston body and additional window/overflow guide in the piston skirt.
Compatibility: The 2023 piston can generally also be used in older POLINI cylinders.
Exception - old cast iron 207 cc cylinders: Cast iron cylinders before 2023 often only have Ø 68.0 mm ex works (≈ 207 cc). The 68.5 mm piston only fits there after boring/honing to 68.5 mm.
Polini differentiates according to cylinder material: Cast iron and aluminum basically use the same piston type, but with material-adapted running clearance tolerance and, above all, different ring tipping.
The decisive difference: Ring configuration according to cylinder material
Note: Rings are crucial
The basic design of the piston is similar - the piston rings must match the cylinder material.
Aluminum cylinder (coated raceway) = coated rings
Cast iron cylinder = uncoated rings
Piston/rings for aluminum cylinders: Rings are coatedon the raceway side (for Nikasil/coated raceways).
Piston/rings for gray cast iron cylinders: Rings are generally uncoated(suitable for gray cast iron raceways).
Practical rule: An aluminum cylinder piston (with coated rings) can also run in a cast iron cylinder without any problems. A cast iron cylinder piston (with uncoated rings) should not be run in an aluminum cylinder, as the coating on the ring side is missing.
Note on tolerance classes for POLINI pistons (aluminum cylinders)
Info: Tolerance classes A-D (aluminum cylinders)
POLINI offers pistons for aluminium cylinders in tolerance classes A, B, C, D. Each class differs by 0.01 mm in piston diameter.
A = smallest dimension (typically standard/ex works) | D = largest dimension (largest dimension within the tolerance chain)
Graduation: B = +0.01 mm, C = +0.02 mm, D = +0.03 mm compared to A
A Nikasil-coated cylinder also wears over its service life. If the running clearance increases, a piston in a higher tolerance class (e.g. B instead of A) can help to compensate for this wear - without having to be recoated directly.
Practical note: Running noise with POLINI aluminum cylinders
POLINI gives its aluminum cylinders a relatively generous running clearance tolerance ex works. This can lead to the pistons making audible running noises under load - even if everything is technically within limits. In many cases, this noise can be reduced with a slightly larger piston (e.g. B instead of A), even if the cylinder is not yet visibly worn.
Cast iron cylinders: standard and oversize
Note: Cast iron = standard and oversize, no A-D classes
There is usually no A-D tolerance chain for POLINI cast iron cylinders. Instead, a distinction is made between:
Standard size: Ø 68.5 mm
Oversize : Ø 68.8 mm (only after repair)
Important: The cylinder must be drilled and honed for the Ø 68.8 mm oversize piston.
POLINI 2023 Design
CVF overflow system
Ø 68.5 mm
Aluminum: A-D tolerance
Gray cast iron: Std./ÜM
TIP for fitting the piston rings
TIP: Please always fit piston rings carefully. Piston rings made of cast material are only elastic to a limited extent and break relatively quickly.
Some pistons have trapezoidal profiled piston rings (so-called keystone rings), these only fit in one installation position. If the ring does not move freely, please remove it carefully and reinstall it "upside down".
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