The maintenance and operational costs of steam locomotives were much higher than diesels. Annual maintenance costs for steam locomotives accounted for 25% of the initial purchase price. Spare parts were cast from wooden masters for specific locomotives. The sheer number of unique steam locomotives meant that there was no feasible way for spare-part inventories to be maintained. With diesel locomotives spare parts could be mass-produced and held in stock ready for use and many parts and sub-assemblies could be standardized across an operator's fleet using different models of locomotive from the same builder. Modern diesel locomotive engines are designed to allow the power assemblies (systems of working parts and their block interfaces) to be replaced while keeping the main block in the locomotive, which greatly reduces the time that a locomotive is out of revenue-generating service when it requires maintenance.