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This issue comes up quite regularly and results from people fitting pre-pressed gear kits to a Piaggio Zip 50cc 2-stroke resulting in unexpected gearing ratios and a noisy gearbox.
Because you can interchange any matched pair of primary or secondary gears between any Piaggio 50cc 2-stroke engine (except Gilera DNA) without problem and because therefore any aftermarket primary or secondary gear kit will work just fine in any Piaggio based 2-stroke engine, many people mistakenly (although not unreasonably) assume that the same applies for pre-pressed gear kits, unfortunately this is not the case.
To understand why this doesn't work it's important to remember that the Zip has different gears as standard to the other Piaggio engines
as standard the teeth on the Piaggio Zip gears are :
on any other Piaggio 50 (except DNA) they are...
so say you use this primary kit...
Stage6 ready pressed gearkit
the first 2 gears are fine because they are a matched pair, but because it is pre-pressed it also includes the 3rd gear (which is identical to the standard 3rd gear on non-Zip Piaggio/ Gileras) to save pressing your pressing off your existing standard one.
The 'standard' 3rd gear on the S6 kit is 13 teeth which although is standard for most Piaggio is not standard for Zip.
If you fit it this means that the standard Piaggio Zip 15 tooth gear has been replaced with a 13 tooth gear which is no longer a large enough diameter to mate well with the 4th gear. This results in a noisy gearbox with poorly meshing teeth. This is the reason some people report Stage6 gear kits are noisy, but it is actually due to incorrect fitment.
The reason this is confusing is because the 4th gear on both Zip and non-Zip Piaggios are both 48 teeth but it's important to realise that the 4th gear on the Zip has been manufactured with a smaller diameter even although it has the same number or teeth (this doesn't affect the gearing).
The non-Zip 48T 4th gear diameter is 76.4mm to mate with the 13T 3rd gear whilst the teeth on the Piaggio Zip 48T 4th gear have had the peaks machined down to 72.25mm diameter to mate with the 15T 3rd gear.
The only way therefore to use a pre-pressed primary kit with a Zip is to press off your old standard 3rd gear and replace the Stage6 one with it... which whilst is perfectly acceptable defeats the point of a pre-pressed kit !
So to summarise...
You can use any primary or secondary gear kit with any and all Piaggio 2-stroke 50cc engines (except Gilera DNA) without any problem because these all contain a matched pair of primary or secondary gears. But you can't use a pre-pressed gear kit on a Zip unless the kit specifically states it is designed just for Piaggio Zip (I don't believe anyone manufactures one).
(note: this article is an after-note to this previous blog... Piaggio/ Gilera Gear-Up Kit Ratio Calculations)