Primary drive gears can rise or lower the final drive ratio of your VESPA motors gearbox. A higher drive ratio becomes necessary when increasing the cubic capacity of your PX80-150 motor with performance enhancing cylinder kits with a cubic capacity of between 166 and 177cc. A performance enhanced 200cc VESPA motor with a tuning cylinder or at least an expansion chamber exhaust system could also benefit from a taller primary drive gear ratio. The extra performance available ensures that in combination with a taller gear ratio the higher revolutions can be turned into a noticeable top-speed increase. More torque orientated motor setups have then the advantage of a higher cruising speed at lower engine revs., with more miles to the gallon and less risk of overheating and the inevitable motor damage that follows.
The primary drive ratio conversion kits consist of a primary drive gear, spring repair set (apart from the DRT version) and a clutch cog. The gear cogs in this conversion kit are of the straight cut type. This makes them more mechanically efficient than helical or angled primary drive gear cogs. The axial thrust created by the OE PIAGGIO gear cogs against the input shaft bearing is also avoided. A higher ratio primary drive makes all the gears taller simultaneously. If you do not want to strip your motor completely for a similar effect, you can replace the clutch gear cog, on its own. If only individual gears need ratio adjustment these too can be replaced with correspondingly larger or smaller cogs.
After a long period of abstinence both MALOSSI and POLINI have their classic primary drive conversion kits back in production. The 24/63 ratio in the MALOSSI kit offers very tall gearing. Due to the fact that MALOSSI 210cc cylindered motors have enough top end power to enable speeds of up to 135 km/h (over 80 mph) to be achieved, without having to use the infamous white van man as a wind-breaker on the motorway. DRT also have a specialised solution in production: The 23/62 gear cog teeth ratio is designed for the premier division of cylinder kits, beyond 210cc. The 24/62 kit is designed for GS 150 motor projects with performance enhancing peripherals such as an expansion chamber exhaust system or even a QUATTRINI M1XL racing cylinder, to enable a higher top-speed. If these kits are intended to be used in combination with a PIAGGIO COSA II clutch then the corresponding DRT clutch cog will also have to be ordered. Alternatively Worb5 produce a primary drive gear cog kit specifically for use with the PIAGGIO COSA II clutch.
Conclusion: There is hardly a more effective method to provide the correct gearing for your motors specific power output.
SIP-TIP: Always install and replace ALL of the smaller primary drive components during each major service. The securing plates on both sides of the primary drive, input shaft holding nut, tab washer and also the input shaft needle bearings should all be replaced for save and enduring use of your motor. Last but not least do not forget to order fresh gearbox oil, gaskets and all relevant oil seals! If you want to mount a 200cc VESPA motors clutch in your PX80-150cc crank case then you will also need the primary drive from the 200cc motor.
SIP Community user ‚Mick‘ on art.-no. 20301000: „Spot on. Assembled easily.”
SIP Community user ‚Vespisti1‘ on art.-no. 20302000: “I'd not try this unless you fit a Pinasco or Polini 177, with a Sito Plus and a 24/24 SI carb. Benefits: You can save fuel when cruising with ease in 4th gear (a lot of it), and at the same time have an unstopable screaming tractor in 3rd gear capabable of climbing a hill, on a windy morning, with a tall screen and two up at 100 km/h”