All smallframe motors always have the same gear steps (except for three-speed and automatic). A longer transmission ratio is only achieved ex works by so-called primary transmissions of different lengths. These transmission ratios are available in numerous gear steps. How long this transmission ratio should be depends on the available displacement and the power delivery of the engine. In the large class (the cubic capacities from 125 cc upwards) it is not necessary to extend the primary of the 125's (2.54) again. Here, the increased speed of the racing cylinders compared to the standard version provides a substantial increase in speed.
Gearbox 24/61, (2.54) teeth, for Vespa 90 R/SS/125/PV/ET3 /PK50 XL/XL2/PK125/ETS/N/S /XL/XL2 also for Vespa 50-125/PV/ET3 /PK50-125/S/XL/XL2 helical cut teeth
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.
TIPS: 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.
OEM Numbers (for the purpose of comparison) 1106705 - 111666 - 110670 - 245261 - 287922 (PIAGGIO)