N-MP-09515 This is for 150cc Scooters, Go Karts, and ATVs needing an extra push from a 4 valve system (most units have only 2) Maximize your GY6 performance with this 4 valve head kit, guaranteed to put you ahead of the pack. You will feel a more responsive low end and a smooth mid range that pulls hard all the way to the top end. When installing this 4 Valve Head on your GY6 , Some modification is necessary. We recommend having this 4 Valve Head kit installed by a professional small engines mechanic. Also the cylinder head studs will need to be replaced with heavy duty studs which are supplied with the kit. I designed the combustion chamber with a diameter of 59mm so it will mate up with cylinder sizes of 57mm to 61mm. This is the first 4 valve head system is for the most common China designed GY6 150cc engine. This particular head is a Taiwanese produced casting and is of excellent quality as are the hardened valves and dual bearing HIGH-RPM camshaft. This is a good buy if you are testing different transmission systems on the Go Karts which need more power. A great mod interesting mod for the serious GY6 gear racers.