There really isn't a ducted-fan unit for an O.S. Max .40 FP engine. The .40 FP isn't powerful enough to be normally considered for ducted-fan use. The only fans in the .40-.45 size have about a 5-inch diameter fan, and are for very high-RPM racing engines that use a tuned pipe and high-nitro fuels. The .40 FP doesn't turn particulary high RPM, and doesn't benefit much from a pipe and high nitro.