The BSA 500 and 650cc Unit Twins were the top-of-the-range models for BSA from the early 1960s through to the Rocket 3 of 1969. The model range covered every angle, from staid tourer, through off-road racers through to true-road burners. This book has two parts - the first describes the design and development of the range from its inception in 1962 to the final demise of BSA in 1973, detailing the various models produced each year. The second part of the book looks at the practicalities of restoring and running the model, through documenting both owners' experiences and a full restoration of a typical bike. The illustrated record of the restoration of a 1965 Lightning identifies the pitfalls that will confront the restorer and the practical modifications that can be made to provide a reliable mount for today's traffic conditions.