This is the Cylinder and Piston Set for the HPI Nitro Star S-25 This is a matched set that is installed inside the engine.FEATURES: ABC Construction (Aluminum, Brass, Chrome) Four Intake Ports, One Exhaust Port Matched SetINCLUDES: One Piston One CylinderREQUIRES: Assembly into Engine.SPECS: Height of Cylinder: 1.2" (30mm) Inner Diameter of Cylinder: .70" (17mm) Outer Diameter of Cylinder: .80" (20mm) Height of Piston Head: .66" (16mm) Diameter of Piston: .70" (17mm)HPIPart HPI1439