Crankshaft -MAZZUCCHELLI bell-shaped shaft (for diaphragm intake) - 60mm stroke, connecting rod 110mm - Vespa PX200

Mazzucchelli bell shaft 60/110 for Vespa PX200

The PX engine offers both advantages and disadvantages due to its intake controlled by a crank web. In the case of uncompromising tuning, the disadvantages of positioning the intake tract directly above the crankshaft unfortunately outweigh the advantages.

In order to be able to convey the largest possible amount of mixture into the engine, even at high engine speeds, the original rotary valve control was replaced early on by a self-controlling diaphragm valve system.
It is not the type of intake control that enables more power, but merely the larger intake area that becomes available due to the now possible milling of the engine housing. However, the crankshaft rotating in the gas flow remains a disadvantage. A new type of shaft was designed to provide the maximum possible cross-section that makes sense for carburetors above 32 mm: The so-called bell shaft (or mushroom shaft), with the new shape of the crankshaft giving it its name.

The bell-shaped shaft is the logical consequence of the two requirements:

• maximum flow cross-section
• maximum stability
• perfect balancing

Despite its unusual shape for Vespa riders, the shaft has no loss of stability compared to a conventionally shaped shaft. Anyone who has ever looked at the crankshaft of a Ducati will probably guess that this design is not only well thought out, but also makes sense for a high-performance engine.

In combination with an increased stroke of 3 mm, the result is a crankshaft that is perfectly suited to the demands of an extremely powerful PX engine.

In combination with a large diaphragm intake manifold and a carburetor of 35 mm to 38 mm, an engine with the right cylinder/exhaust selection can easily shovel over 35 hp to the rear wheel.

Of course, you can also build beautiful touring engines with such a bell shaft. The advantage here lies in the wide speed range and the better torque curve even at low revs due to the longer stroke and larger displacement.
The very low-vibration running is just the icing on the cake...