Abstract Over the past fifteen years extensive developments have demonstrated the suitability of moulded reinforced polymer composites as replacements for cast and fabricated “bolt-on” engine components in mass produced vehicles and yet little has reached production. The article discusses some of the complex and interacting relationships between material selection, processing options, component design, investment and component costs which influence progress down the route to materials substitution. In a review of the progress already made, the problems resolved and those remaining, it is suggested that the next few years should see this work reach commercial fruition.