A field survey of diesel engines in light commercial trucks showed that the failure profile could be described by an early failure region, indicative of manufacturing quality, and an exponential distribution, representing the reliability of the engine, merging into a log-normal wear-out region. An overall reliability of 0. 9 after a year' s service was apparently required. Engine availability could be improved significantly, in a cost effective manner, by the selective upgrading of components and sub-assemblies according to their criticality.Conventional design criteria do not recognise the need for modifying the failure risk of individual parts and assemblies according to their likely impact on the operational behaviour of the engine as a whole. An alternative methodology was derived, therefore, as part of the design and validation of a 5. 8ℓ(354 in3) engine intended predominantly for the N. American Class VI truck market. The development of this revised design and development approach is described.