15mm retaining ring for original Vespa PK 80 /100 /125 and PV 125 /ET3 pistons.
There are two different grooves on pistons - square and round. The original Piaggio cylinder set has a square-machined locking groove, and its piston pin is secured with a square retaining ring.
Tuning cylinders from DR, Polini, Malossi, etc., typically have a round cutout. Their piston pin is then secured with a piston circlip.
Piston clips with the square shape - thus "circlips" - are mounted / dismounted with a circlip plier.
Note: The illustration shows a 12mm retaining ring.
