Vespa PX 24 teeth clutch gear - No whine


First of all, sincere apologies If some of the things I write are not clear, I am new to classic Vespa world and do not really understand most terms, and it is quite overwhelming to try to figure it out.

I have a Vespa PX 125 ZAP M74100 with a Polini 177.

I asked the dealer to modify the gearing as it was too short after the kit conversion. He changed the pinion clutch gear for a 22 teeth model, but I now have a whine from the transmission due to the pinion having straight teeth. As far as I know, nothing important was changed except for the clutch pinion. 

Apart from the noise which I can live with, even though I would prefer without it, I would like to confirm that this was the correct part to use.

Is there another clutch pinion gear we could have used with helical teeth instead ? Would I need to get a combo of clutch pinion gear and corresponding primary gear instead ? I see a lot of references about the COSA 2 clutch....does my bike have this as stock ? 

Thank you for your help [:)]


Hey Werner,


Thank you for your reply, much appreciated !

As far as I know, the dealer only changed the clutch pinion gear for a 22 teeth one, but did not change the primary. There is only one part referenced on the invoice. Are the gears that came on the scooter originally straight or helical ? My understanding is that they mixed a straight 22 teeth clutch pinion with whatever was on the scooter stock and this caused the whine.

As for the clutch, the scooter broke down on the way to work this morning. It sounded like a nasty screeching noise like there was a loose part in the gearbox, immediately followed by a thud.Engine stalled and won't turn. Kickstand won't budge. I did have oil so I am convinced I did not cease. I strongly suspect that they forgot a washer or something inside the engine as I started having this screeching noise after the Polini install and it only would make the noise occasionally when I was going through turns. Of course, when I called them up to inquire about the noise they said it was normal [:@] 



As per your description, the dealer took the engine apart and changed the entire primary gearbox using a straight tooth combination.


So far so good, there's nothing wrong with that. I am sure you will hear a wail. In your case, however, I would consider it the right decision as the Polini 177 exposes the motor to higher forces (gearbox, ball bearings, etc.).




The basic differences between straight and helical teeth are as follows:



  • straight teeth transmit the torque better (depending on the geometry) than the helical gearing.




  • Straight teeth do not exert any axial forces that negatively affect the main ball bearing of the crankshaft than is the case with helical gearing.



I am surprised why the dealer did not offer you a better clutch, as a 125cc engine only comes with one that is too small when paired with a Polini 177.


The best would be a SIP Ultrastrong clutch (fundamental advantage: CNC-milled from one piece of metal, torsion-resistant, without rivets that loosen after a while ... also a better, hook-free geometry of the inner mechanism.

Sorry, but straight teeth cannot be mixed with helical teeth. That is physically impossible.
Originally the PX is equipped with helical theeth.

I also cannot tell about that dealer or what exactly happend to your engine (as I don't have a cristal ball, sorry!), but it sounds to me that you have to insist the dealer to again take a look at it or you find a more competent mechanic.

For me it sounds as if the clutch or some part of it has gone loose, and then beeing crushed by the gearbox. Nothing you want, nothing that would be normal.
On the first instance you could check the gearbox oil (let it flow into a clean vessel) and see if there is anything to discover, metal parts or chippings for example. 

It would be interesting anyways of what positions exactly your invoice consists of and how much you paid for it, and then...what would eventually be normal or not.