The earliest choppers tended to be based on Harley-Davidson motorcycles, at first making use of the Flathead, Knucklehead and Panhead engines—many of which could be found in surplus military and police motorcycles bought cheaply at auction. As new engines became available they were soon utilized in choppers. British bikes, particularly Triumphs, were also a popular motor for choppers early on. As the Japanese manufacturers began offering larger engines in the late 1960s these motors were also quickly put to use by chopper builders. The Honda 750-4 was the most widely used Japanese motor for chopper builders early on. Choppers have been created using almost every available engine, but builders have always shown a preference for older air cooled designs. It is rare to see a chopper with a radiator.