A hybrid elastomer-and-metal spring isolator (20) with a low natural frequency. A fully bonded sandwich mount (22) has cored pockets (28) that receive metal coil springs (30). Extruded cavities (32) on the lower (26L) and upper (26U) plates of the sandwich mount (22) extend into the pockets (28) within the interior of the coiled springs (30) and receive a tension bolt (40) and double lock-nut assembly (42) to secure the separate lower cavities to the isolator. A preload retention nut (48) engages a protruding portion of the tension bolt (40) and preloads the coil springs (30) to an amount equal to their static load. This hybrid isolator (20) has particular application in mounting large (30 ton) diesel engines. The isolator assembly provides a natural frequency that is between 3 and 5 Hz, outside the normal idling frequencies of the engine so as to avoid resonance.