POLINI Racing Vespa T5 152 Piston KitPOLINI Racing Vespa T5 152 Piston Kit Piston rings made of cast material must be mounted carefully as they have limited elasticity and are prone to breaking. If using trapezoidally profiled piston rings (also known as keystone rings), ensure they are installed in the correct position as they only fit in one way. If the ring is not free, carefully loosen and reinstall it upside down. diameter: 16 mm length: 50 mm Cylinders bore: 61 mm Comparison No.