The gy6 engine is a designation used for an engine design installed on motor scooters produced in some Asian countries. The component, however, is also used for motorcycles and three- or four-wheelers known as all-terrain vehicles (ATVs). The gy6 engine is available in a variety of specifications.

The origin of the gy6 engine, and even the meaning of the term "gy6," is mired in mystery. Some people believe that Japanese automobile/motorcycle manufacturer Honda came up with the design in the 1980s. This may be based on the fact that Honda's Elite and Spacey line of scooters once sported the gy6 engine. There is no known gy6 configuration in the company's scooters from the 1980s era, though. Instead, the manufacture can be traced to scooters produced in Taiwan and China, but the exact date and manufacturers involved are largely unknown.

The gy6 engine is made as a four-stroke single-cylinder with two overhead valves. There are three sizes available: 50 cubic centimeters (cc) or 3.05 cubic inches (ci), 125 cc (about 7.63 ci), and 150 cc (about 9.15 ci). The revolution-per-minute (rpm) range is 7,000 to 7,500. The 150 cc is the most powerful and popular one, with the ability to produce up to around 9 horsepower (hp). Owners of gy6 engine-equipped vehicles, however, can make modifications to increase horsepower up to 14 hp.

Also, gy6 engines are manufactured in a short case or long case style. The difference in swing-arm lengths allows for end users to mount 10-inch (25 cm) tires and rims on the former, and up to 13-inch (33 cm) tires and rims on the latter. In some cases, end users have been able to add tires and rims as large as 16 inches (about 41 cm) due to extra-long variants of the engine.

Regarding its cooling system, the gy6 engine uses water or oil. With the former, water is used as the heat-removal agent, while with the latter, oil carries the heat through a cooling unit such as a radiator. The engine uses a capacitor discharge ignition (CDI) for its electronic ignition system, with a magnetic trigger fitted on the crankshaft.

Taiwan and China are the premier manufacturers of the gy6 engine. Examples of Taiwanese motor scooter, motorcycle and ATV brands include Adly, PGO, Kymco and SYM. Chinese brand examples include Neco, Qianjiang, Xingyue and Zongshen. Some of these gy6-equipped brands are exported outside the Asian continent. For instance, the United States imports the vehicles of Chinese company PGO as the Genuine Scooter Company, and it has a division of the Taiwanese manufacturer Kymco.