Modern Vespa

Just bought a GOL replacement piston regular size for Vespa gs160. Original bore is 58mm. Got some wear but not ready to rebore. The regular 58mm piston should be 57,89mm (~.015) according to original Piaggio documentation. The new GOL piston is 57,60mm out of the box. So way under spec. Wich would mean piston to cylinder clearance of .40mm wich would be far over the recommended clearance of .12 to .18mm. Anybody experience this discrepencies with GOL piston? Does the metal composition of those piston expend more under heat hence the undersized, or is it just a badly made, or cheap, piston?

Personally I'd go with the Grand-Sport piston if you can find one.

Thanks for the suggestion, but It would make absolute no sense to me to scrap a rare 160gs cylinder that was never re bore, just to put a big piston that would'nt allow any future re bore, who also suffer of part availability problem. I'm looking for a good piston within spec, or at least an explanation on why GOL piston are so far off, that can help me understand.

GOL pistons are shit. Reboring to 1st o/s will not scrap a stock cylinder if you have no other options. You have three more rebores after that…

I see that my supplier has NOS G.C.M. pistons in limited stock, you could contact him, he ships abroad…

https://www.scootershop.gr/product_info.php?products_id=6327

GOL is shit, I bought one to replace my P200 that had scratches, the hole for the pin was too high, which ends up the bottom touching the crank... I removed enough material at the bottom to avoid that and finally discard that shit, don't want to take stupid risks. I just smoothed the scratches of the original piston instead and replaced the rings.

Return it while you can

The previous suggestion was to use the grand-sport wich is 62mm (190cc)! Wich explains my response. That's why I plan to use the regular small step over bore if I need to, and not jump from 58 to 62.

Thanks for the link of your guy will get in touch.

Are those GCM piston as good as PIAGGIO original ? Have you ever used them ?

Grand-Sport did make pistons for the 160 from 58-60mm, 5 years ago I got a 60mm to clean up a cylinder with exhaust port damage.

I opened up the windows, could not help myself.

Unfortunately only the big bore is still available, and I'm not interested for the reason mention above.