A tip-turbine engine comprises a fan-turbine rotor assembly that includes one or more turbine ring rotors. Each turbine ring is assembled from a multitude of turbine blade clusters. By forming the turbine blades in clusters, leakage between adjacent blade platforms is minimized which increases engine efficiency. Assembly of the turbine blade clusters to a diffuser surface includes axial installation and radial locking of each turbine blade cluster. This is accomplished by attachment lugs protruding from the underside of the arcuate base of the turbine blade clusters. Torque load surfaces are also integrated into the arcuate base.