Piaggio Vespa Piston Kit (Standard Size 57.8mm) PX150, COSA 150Piaggio Vespa Piston Kit (Standard Size 57. 8mm) PX150, COSA 150 Standard Piston to fit the Vespa 150 Models. When refitting pistons, it's best to check that all parts are clean and fit into to the barrel. Checking to make sure that the circlips are fitted correctly into the cut out inside of the Piston assembly. 57. 8mm 2 piston ring(s) standard Comparison No. 2441730001 (PIAGGIO)