Gearbox 29/68, (2.34) teeth,
for 135-155cc cylinder, DRT
RACE, for Vespa 50-125/PV/ET3
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 effects 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 and the larger around the outer edge of the clutch basket. The gear cogs are almost exclusively straight cut items. They lose less energy than 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, you can just change the primary drive for the same effect.
DRT have equipped these sport primary drive units with their very own CNC machined spring retaining plates that have a more sturdy construction and also stiffer springs so offering a simple primary drive plug-and-play alternative to a complete primary drive service.
The DRT race primary drive unit includes the in special steel CNC machined primary/clutch basket produced by DRT. This high-end kit is very popular on the Italian race scene as it is a dedicated race product and not really suitable for every day road use.
Conclusion: If you are looking for a reinforced primary drive unit then look no further.
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)
IPs13rdpz68.26k/700 Rac. (DRT)
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