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Gearbox 24/72, (2.95) teeth,
for 75-102cc cylinder, POLINI
Sport, for Vespa 50-90/R/SS
straight teeth, with aluminium
The usual method of altering the overall gear ratio of a small frame motor is by changing the primary drive. A taller primary drive really starts to make sense with all cylinder kit conversions over 75cc. Principally speaking, the stronger the motor, the taller the overall gear ratio should be.
The gear ratio produced by the primary drive affects the gear ratio of the whole motor (overall gear ratio) they comprise of 2 gear cogs in the small frame motor, the 1st smaller one, located on the crankshaft, it´s larger counterpart around the outer edge of the clutch basket. The gear cogs are almost exclusively straight cut items. They lose less energy than diagonally cut (synchromesh) versions and are so more efficient creating virtually no axial thrust on the bearing of the input shaft/ clutch. If you require a higher overall gear ratio for your motor setup but do not want the hassle of taking the motor apart completely, just change the primary drive for the same effect. The details listed at our website of which ratios suit which classes of cylinder are reliable and well proven guidelines compiled over many years.
The primary drive gear kits produced by RMS are very good value-for-money. The clutch baskets produced by OLYMPIA are very solid in construction and the SURFLEX version is preferred by the DRT riders. The kits produced by POLINI fill the gaps in the available gear ratios left open by the original PIAGGIO items. Compared to most other versions in production their high end aluminium clutch basket can also save you a lot of weight. A complete set of clutch plates is also included.
Conclusion: There is hardly a more effective method to raise the gear ratio of your motor in order to match its increased power output.
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.
OEM Numbers (for the purpose of comparison)
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