Extend gearbox on Vespa PK 50 / V50 without splitting the engine

The gear ratio can be adjusted by changing the primary gear. When changing the primary gear, both the small and large sprockets are replaced.

The highly popular tuning cylinder DR 75 for the Vespa V50 and PK 50 S-XL2 benefits greatly from a longer gear ratio because it makes the first gear useful again and allows the existing torque to be effectively converted into forward motion. The maximum speed also increases significantly with a longer gear ratio for the DR 75, as explained in the article on choosing the right primary gear.

When installing the tuning cylinder, many people refrain from lengthening the gear ratio because changing the entire primary gear, i.e., the small and large sprockets, is only possible by splitting the engine. An exception to extending the gear ratio is this specially designed sprocket, which has more teeth but can be paired with the original sprocket. Only the clutch cover needs to be removed, and the original small sprocket with 14 or 15 teeth must be replaced with the new sprocket with 16 teeth. However, this only makes sense for cylinders from 75 to 85 cc.
The RPM level remains high. The 75 cc cylinders have enough material to absorb heat without it becoming critical too quickly.

From a cylinder size of 102 cc, a complete change of the primary gear becomes necessary.