ScootsUSA 164-302-8318 150cc GY6 Complete Cylinder Head - Emission Complete Emissions Type 57mm Cylinder Head for 150cc GY6 4-stroke QMJ157 engines.Emissions Type a.k.a. PAIR is an acronym for Pulsed Secondary Air Injection System. It is an emissions control system used to lower HC and CO emissions. It works by allowing air into the exhaust flow. Emission Control System: Pulsed Secondary Air Injection System (PAIR)Overall Height: 60mm Bore Diameter: 57mm Intake Port Diameter: 24mm Exhaust Port Diameter: 24mm