In 1939, with involvement in World War Two imminent, Indian began developing a 30.5ci (500cc) version of the Sport Scout called the Model 640 . The engine was derived from that of the Junior Scout with a lowered compression ratio, mild valve timing and other modifications designed to ensure reliability. It featured a T-shaped inlet manifold instead of the Sport Scout’s Y-shape and an oil bath air cleaner was added. Ground clearance was increased by fitting longer front forks and modifying the rear frame and the mudguards were mounted well clear of the wheels to prevent clogging with mud.