During the 1920s there was rapid development in the design of diesel engines. In 1929 a Gardner "4L2" marine engine was fitted into a Lancia bus. This conversion was successful and prompted Gardner to introduce the "LW" series of diesel engines, designed especially for road vehicles but later modified and supplied as a marine engines with factory-fitted bilge pumps. The LW engine was a modular design, with separate cast iron cylinder blocks and cylinder heads comprising either 2 or 3 cylinders. A 5-cylinder engine would thus use a "2" and a "3", whereas a 6-cylinder engine could have either 3 "2"s or 2 "3"s. Boat engines had a cast iron crankcase, whereas (in the interest of lightness) road vehicles would have an aluminium alloy crankcase. Any boat engine with an alloy crankcase would be a marinised road engine.