Vertex Piston Kit, Athena small end bearing and Mitaka top end gasket set. Fits BETA RR250 2018-2025. Graded size 66.35 A. The clearance is to allow for piston expansion when it heats up. Vertex pistons are coated with a special Molybdenum Bisulphite black coating to aid during piston break in and reducing friction during the work inside the cylinder.
