Various online retailers such as Golden Eagle Bike Engines and Bicycle-Engines sell four-stroke bicycle engines at their online stores. Used bicycle engines and bicycle engine kits can also be found at online auction sites such as eBay.
Four-stroke bicycle engines are small engines that are used to turn a manual bicycle into a motorized one. The engines either use the bike's chain or use friction to power one of the wheels. Full bicycle engines can be bought from the online stores of companies that manufacture them, such as Golden Eagle Bike Engines, directly from their website. Similarly, used engines can be found at online auction sites or from online classified ads within the bicycling community.
While it is possible to buy four-stroke bicycle engines as a whole, it is much more common for individuals to buy the engines as a do-it-yourself kit. This is because all bicycles are different and it is typically easier to assemble the engine around the bicycle instead of trying to fit a full engine onto a bike. Four-stroke bicycle engine kits are available for purchase through online companies such as Bicycle-Engines, and they come with instructions on how to assemble the motor around different types of bicycles to turn the bike into a motorized vehicle.