Piston Piaggio Original – 50cc Hi-PER4 AC 4V – Vespa LX, Primavera, S, Sprint – Category A, B, C, D
Original Piaggio pistons for air-cooled 50cc four-stroke engines from the Hi-PER4 range with four valves. The pistons are designed for rebuilding standard cylinders and are available in four size categories. Each category has its own Piaggio original part number, allowing the piston to be matched to the corresponding cylinder size classification.
The pistons in the 8734660001 to 8734660004 series are used in Vespa LX, Primavera, S and Sprint models, among others, as well as selected Piaggio and Aprilia models. The specific engine version and required piston category are decisive when selecting the correct replacement part.
Piaggio Hi-PER4 – the four-stroke engine family
Hi-PER4 is the designation used by Piaggio for a family of compact four-stroke engines. This designation distinguishes this engine generation in particular from the Hi-PER2 range of two-stroke engines.
Hi-PER4 engines were developed for various scooters from Piaggio, Vespa and Aprilia. Their construction is designed for economical everyday operation, compact dimensions and the requirements of small engine capacity classes.
The Hi-PER4 engine family includes different technical versions. These include engines with two valves as well as the four-valve version relevant here. The Hi-PER4 designation alone therefore does not automatically mean that an engine has four valves.
The 50cc Hi-PER4 4V engines feature a cylinder head with four valves. Two intake valves allow the fresh charge to enter the combustion chamber, while two exhaust valves discharge the combustion gases.
Using two intake and two exhaust valves allows the flow cross-sections and gas exchange processes to be specifically designed. This enables a cylinder head configuration adapted to the requirements of a small four-stroke engine.
Valve operation is controlled by an overhead camshaft driven by a timing chain. The engine therefore uses a mechanical valve train in which the opening and closing movements of the four valves are synchronized with crankshaft rotation.
Technical design of the 50cc Hi-PER4 4V engine
The Piaggio Hi-PER4 50 4T 4V is an air-cooled, single-cylinder four-stroke engine with an actual displacement of 49.9cc.
The standard cylinder bore is 39mm. Together with a piston stroke of 41.8mm, this produces the displacement intended for this engine class.
Cooling is provided by an airflow generated by the engine cooling fan. This airflow is directed over the cooling surfaces of the cylinder and cylinder head, carrying away the heat generated during operation.
Engine lubrication is provided by an oil circulation system. The moving engine components are supplied with lubricating oil to reduce friction and wear.
The corresponding scooters use a continuously variable automatic transmission with a variator and centrifugal clutch to transmit power. The engine design is therefore adapted to the automatic drivetrain of small scooters.
On the Hi-PER4 4V models listed here, the air-fuel mixture is prepared by a carburettor. This design differs from later generations of Piaggio 50cc engines equipped with electronic fuel injection.
Piston categories A, B, C and D
Piaggio uses four different size categories for its original 50cc Hi-PER4 4V pistons. These are identified by the letters A, B, C and D or the numbers 1, 2, 3 and 4.
The categories account for small differences within the standard manufacturing tolerances of pistons and cylinders. They allow both components to be matched correctly without representing conventional oversize repair pistons.
The four versions correspond to the following Piaggio original part numbers:
- Category A / 1 – Piaggio original part number 8734660001
- Category B / 2 – Piaggio original part number 8734660002
- Category C / 3 – Piaggio original part number 8734660003
- Category D / 4 – Piaggio original part number 8734660004
Piston diameters of 38.95mm, 38.96mm and 38.97mm are specified for categories A, B and C respectively. These dimensions refer to the individual piston versions and must not be confused with the engine's nominal bore of 39mm.
The correct piston category must be determined from the cylinder marking and, where necessary, by accurately measuring the cylinder bore. Selecting a piston solely on the basis of the vehicle model or year of manufacture is not sufficient.
Construction and visible features of the original piston
The Piaggio piston features a piston crown shaped specifically for the four-valve engine. Several valve reliefs are visible on the piston crown, corresponding to the arrangement of the valves in the cylinder head.
The shape of the piston crown forms part of the standard combustion chamber geometry. It takes into account the design requirements for valve clearance and the intended configuration of the combustion chamber.
The upper surface shows machined areas as well as sections with a clearly visible cast surface texture. Additional markings are present on the piston crown and must be considered when identifying the piston and determining its correct installation orientation.
The grooves for the piston rings are located in the upper section of the piston. The piston ring arrangement seals the combustion chamber and regulates the lubricating oil film on the cylinder wall.
The piston skirt has a machined sliding surface that guides the piston inside the cylinder. The side of the piston contains the piston pin bore, which provides the connection to the connecting rod.
The corresponding piston pin has a nominal diameter of 13mm. The components actually included with each product variant must be checked against the specific scope of delivery.
Technical specifications
- Manufacturer: Piaggio
- Product: original piston
- Engine family: Hi-PER4
- Engine version: 50cc 4T AC 4V
- Engine type: single-cylinder four-stroke engine
- Actual engine displacement: 49.9cc
- Nominal engine bore: 39mm
- Engine stroke: 41.8mm
- Number of valves: four
- Valve configuration: two intake valves, two exhaust valves
- Valve train: overhead, chain-driven camshaft
- Cooling: forced-air cooling (AC)
- Fuel system: carburettor
- Piston pin diameter: 13mm
- Piston crown: contoured with valve reliefs
- Available size categories: A, B, C, D or 1, 2, 3, 4
- Piston diameter category A / 1: 38.95mm
- Piston diameter category B / 2: 38.96mm
- Piston diameter category C / 3: 38.97mm
- Piaggio original part number category A / 1: 8734660001
- Piaggio original part number category B / 2: 8734660002
- Piaggio original part number category C / 3: 8734660003
- Piaggio original part number category D / 4: 8734660004
- Application: replacement in the event of wear, damage or engine rebuilding
Suitable for Vespa
- Vespa LX 50 4T 4V (2009–2013)
- Vespa Primavera 50 4T 4V (2013–2017)
- Vespa S 50 4T 4V (2008–2012)
- Vespa Sprint 50 4T 4V (2014–2018)
Suitable for Piaggio models
- Piaggio Fly 50 4T 4V (2012–2013)
Other compatible models
- Aprilia Scarabeo 50 4T 4V (2009–2017)
- Aprilia SR Motard 50 4T 4V (2013–2019)
Vehicle compatibility applies exclusively to the corresponding versions equipped with a 50cc Hi-PER4 four-stroke engine with four valves. The engine variant and required piston category must be checked before ordering replacement parts.
Important information on selection and engine rebuilding
Before installing a new piston, the cylinder bore must be inspected for wear and damage. This includes checking for scoring, examining the cylinder geometry and measuring the actual internal diameter.
The correct combination of piston and cylinder is essential for maintaining the specified running clearance. Insufficient clearance can lead to increased friction or piston seizure as the components expand with heat.
Excessive clearance, on the other hand, can impair piston guidance and cause increased operating noise as well as reduced sealing performance.
During installation, the specified piston orientation, correct positioning of the piston rings and their ring end gaps must be observed. The piston pin and its retaining clips must also be installed correctly.
The existing size category must not be replaced with another category without verification. The four versions are intended for standard manufacturing tolerances and are not repair pistons for cylinders that have been bored out to a significantly larger diameter.
Original Piaggio replacement part for 50cc Hi-PER4 four-valve engines
The original Piaggio pistons 8734660001, 8734660002, 8734660003 and 8734660004 allow targeted piston replacement in the corresponding 50cc Hi-PER4 4V engines.
The four size categories allow the replacement piston to be matched to the existing cylinder classification. This preserves the combination of piston geometry, cylinder bore and running clearance intended for the standard engine, provided that the components remain within the permissible wear limits.
An original Piaggio replacement part for professional rebuilding of the corresponding Vespa LX, Primavera, S and Sprint engines, as well as other Piaggio and Aprilia models equipped with an air-cooled 50cc four-valve engine.
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