Approximately 2 minutes after takeoff, the pilot and passengers heard a loud muffled boom from behind the firewall, then the rotor RPM began to decay. The pilot entered an autorotation and selected a landing area. He noted that the engine was not driving the rotor system, so he shut it down to reduce the possibility of fire while landing. During an emergency landing on sloping, mountainous terrain, the helicopter rolled over, and subsequently, it came to rest in a ravine at the bottom of the slope. A teardown of the engine revealed that the #3 bearing, part number 4-301-051-02, had failed and that the oil jet for the bearing was clogged. Reportedly, maintenance personnel were aware that a differential pressure check, to detect clogging to the #2 and #3 packaged oil jets, was 7 to 8 psi. This was 2 to 3 psi too high and indicated clogging of the oil jets. (This procedure was outlined in Lycoming Service Bulletin LTS101A-17-0014). However, the discrepancy was not entered in the engine maintenance logs.