VMC RACING Solid-Flange Crankshaft for Vespa Smallframe with Diaphragm Intake – 45 mm stroke, 87 mm CNC connecting rod, Ø20 mm taper, and weighing over 1.5 kg.
The VMC RACING solid-flange crankshaft is designed for performance-oriented Vespa small-frame engines with a diaphragm intake. With a 45 mm stroke, an 87 mm-long CNC-machined connecting rod, and a 20 mm diameter cone, it provides a suitable foundation for compact, high-performance setups such as the VMC RACING RVA 100 cc.
The solid-flange design is particularly well-suited to a diaphragm engine: Since the intake air is controlled by the diaphragm, the crankshaft does not need to feature a traditional rotary valve control surface. This allows the crank flanges to be designed as solid pieces.
VMC specifies a weight of over 1.5 kg for the crankshaft. The comparatively high rotating mass is well-suited for a powerful street or sport engine, where controlled engine operation and smooth power delivery are essential.
45 mm stroke 87 mm connecting rod Solid flange Ø20 mm taper >1.5 kg Diaphragm inlet VMC RVA 100 Vespa Smallframe
In a conventional rotary vane motor, the crankpin handles part of the intake control. In a diaphragm motor, however, this task is performed by the diaphragm. It opens and closes depending on the pressure conditions in the crankcase.
This allows the crankshaft to be designed with solid webs. The closed-web design is particularly suitable for engine configurations in which intake control is handled entirely by a diaphragm block.
The crankshaft operates with a 45 mm stroke. Compared to a stock small-frame crankshaft, the additional stroke—in combination with a cylinder designed for this purpose—can increase displacement and torque potential.
It is always important that the cylinder, piston, connecting rod length, and crankshaft stroke are compatible as an integrated system. This crankshaft is intended, among other things, for use with the VMC RACING RVA 100 cc engine.
VMC equips the crankshaft with an 87 mm long, CNC-machined connecting rod. The connecting rod length is an essential component of the engine’s geometry and influences the position of the piston within the cylinder.
When building the engine, the stroke and connecting rod length should therefore not be considered separately. The combination of a 45 mm stroke and an 87 mm connecting rod must match the geometry of the cylinder being used.
According to VMC , the crankshaft weighs more than 1.5 kg. This gives it a comparatively high rotating mass for a small-frame crankshaft.
A higher flywheel mass can contribute to smoother and more even engine operation and positively influence power delivery under load. However, the extent of this effect in a specific engine depends on the overall setup, including the flywheel, clutch, cylinder, and gear ratio.
According to the manufacturer, the two crank arms have diameters of 82 mm and 86.5 mm, respectively.
An important advantage of this design: According to VMC, the engine block does not need to be bored out to accommodate the crankshaft journals. This eliminates the need for extensive machining of the engine block, which might otherwise be necessary with larger crankshaft diameters.
The crankshaft features a Ø20-mm cone on the ignition side. This makes it compatible with corresponding small-frame ignition and magneto solutions.
Before installation, verify that the pole wheel or ignition system is compatible with the crankshaft variant being used.
VMC recommends using wider needle bearing cages for this crankshaft than those found on the original Piaggio version.
Since the manufacturer does not specify exact bearing dimensions in the provided information, the needle bearings used should be selected to match the pistons, piston pins, and connecting rods.
A typical application for this crankshaft is use in the VMC RACING RVA 100 cc engine. The combination of a diaphragm intake, full-flange crankshaft, 45 mm stroke, and 87 mm connecting rods forms a compact foundation for a performance-oriented small-frame engine.
Of course, the crankshaft can also be used in other engine setups, provided that the stroke, connecting rod length, taper, and other geometric requirements match the respective cylinder and engine design.
The crankshaft is intended for compatible Vespa small-frame engines. In addition to the vehicle model, the actual engine installed, as well as the cylinder and ignition combination used, are always decisive factors.
| Manufacturer | VMC RACING |
|---|---|
| Component | Crankshaft |
| Design | Solid web |
| Intake Design | For diaphragm inlet |
| Stroke | 45 mm |
| Connecting rod length | 87 mm |
| Connecting rod | CNC-machined |
| Taper | Ø20 mm |
| Crankcase | Ø82 mm / Ø86.5 mm |
| Weight | Over 1.5 kg according to the manufacturer |
| Crankshaft housing machining | According to the manufacturer, no boring of the crankcase is required for the crankpin |
| Needle bearings | The manufacturer recommends a wider version than the original Piaggio design |
| Manufacturing | Made in Italy |
| Typical application | VMC RACING RVA 100 cc and comparable diaphragm setups |
| Vehicle Group | Vespa Smallframe |
| Models | Vespa V50, PK50 with corresponding engine configuration |
The VMC RACING 45/87 full-flange crankshaft is a crankshaft consistently tailored to performance-oriented Vespa small-frame diaphragm engines.
With a 45 mm stroke, 87 mm CNC-machined connecting rods, a 20 mm diameter taper, and a weight of over 1.5 kg, it offers an attractive foundation for compact and powerful setups such as the VMC RACING RVA 100 cc.
A particularly practical feature is that, according to the manufacturer, the crankshaft journals—with diameters of 82 mm and 86.5 mm, respectively—can be installed without having to bore out the engine case. Thus, the crankshaft combines a performance-oriented solid-journal design with a comparatively manageable machining effort.
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