In the engines of smallframe VESPA models the power created by the motor is transmitted by the primary drive cog, located on the crankshaft, to the primary driven cog located on the outer edge of the clutch basket. The clutch assembly is located on the input shaft of the motor itself which transmits the power further through the output shaft to the rear wheel. The more power the motor puts out, the sooner your clutch needs replacement.
The original primary drive has diagonally cut (syncromesh) connecting teeth and are delivered including the primary drive cog and clutch basket with the corresponding cog around the outside. If a clutch basket from another model of small frame VESPA is used this can make the overall gear ratio of the motor either taller or shorter. If the model you are looking for is not listed it is only available with straight cut gear cog teeth. Through changing the size, or number of teeth, on the primary drive you can vary the overall gear ratio of your motor.
Due to the fact that they have differing outer diameters these cogs are not automatically compatible with all other sizes of cogs available. The best solution is offered in the form of the DRT range of engine gear cogs...
Conclusion: The primary drive assembly should be replaced during every major service of your gear box.
SIP-TIP: Do not forget to order a new fixing nut for the input shaft, a safety washer, new bearing and fresh gearbox oil for safe and enduring use of your motor.