"Sectioned vertical twin-cylinder 500cc engine. The prototype was designed by Edward Turner, and went into production in 1938. The engine has two parallel cylinders contained in one finned casting, overhead valves, presure pump lubrication and a single carburettor. The aim in the design was to produce a compact and more powerful engine, with quieter, easy starting and smooth running. Triumph's Speed Twin is regarded by many as the definitive British motorcycle."
Via: Science and Society Picture Library