The FAA is adopting a new airworthiness directive (AD) for Teledyne Continental Motors (TCM) models 520 and 550 series engines with certain ECi cylinders installed. This AD requires replacing certain serial-numbered (SN) cylinders supplied by ECi before further flight. This AD results from reports of 34 failures of cylinder heads marketed by ECi. We are issuing this AD to prevent loss of engine power due to cracks in the cylinder head and possible engine failure caused by separation of a cylinder head.