On June 17, 2010, at 1900 eastern daylight time, an amateur-built McCulloch RV-4, N418GM, lost engine power while transitioning over the city of Pageland, South Carolina. The pilot made a forced landing on a road, and the airplane was substantially damaged. The airplane was registered to and operated by a commercial pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan was filed. The commercial pilot received minor injuries. The flight departed Peachtree DeKalb Airport (PDK), Atlanta, Georgia, at 1800. The pilot stated that during cruise flight the engine made a “popping” sound and started to lose power. He contacted air traffic control, declared an emergency, and then made a forced landing on a road. During the landing, the airplane collided with a ditch and hit a road sign. Examination of the airplane by a Federal Aviation Administration inspector revealed that the airplane landed on a local highway. Examination of the airframe and flight control system components revealed no evidence of preimpact mechanical malfunction. Examination of the engine revealed that valve train continuity could not be established throughout the engine. Examination of the fuel system revealed that both fuel tanks were nearly full of fuel. Further examination of the fuel system revealed that no water or debris was found in the fuel system. The wreckage was recovered and retained by the NTSB for further examination. Examination of the experimental engine by the NTSB revealed that valve train continuity could not be established throughout the engine. Further examination of the accessory housing revealed that the right magneto was replaced with a crankshaft sensor module and a Light Speed Engineering plasma capacitive discharge ignition system. This system was connected to the upper spark plugs; and when the crankshaft was rotated, spark was noted to the upper leads and spark plugs. The magneto was removed and wear marks were observed on the magneto gear and the engine case. Examination of the left magneto revealed that when the crankshaft was rotated, no spark was observed on the lower plugs. The left magneto was removed and when rotated, spark was observed on the magneto leads ends. The accessory housing was removed to access the accessory gears for further examination. This revealed that the upper crankshaft idler gear was not bolted to the case. Further examination revealed that the bolt and the case bolt holes were elongated and stripped. A review of the Lycoming parts manual pages revealed that the crankshaft idler gear shaft was missing a lock plate. During an examination of the oil sump and all areas within the engine case, the lock plate was not located. No other anomalies were noted during the engine examination. A review of the engine maintenance records revealed that the engine was overhauled by Triad Aviation Incorporated on September 3, 2003. On June 7, 2010 an annual conditional inspection was conducted on the engine, and 320.74 hours since overhaul was recorded. There were no other entries were made in the logbooks after the overhaul that addressed the dismantling of the engine.