On February 26, 2000, about 1620 hours Pacific standard time, an Ercoupe 415-C, N99590, was substantially damaged when it force landed in a field and collided with a fence at San Bernardino, California. The airplane was owned and operated by the private pilot who was not injured. Visual meteorological conditions prevailed for the personal flight operating under 14 CFR Part 91. No flight plan was filed. The flight originated at Riverside, California, at 1600, and was destined for Apple Valley, California. The pilot reported that the engine began to vibrate during cruise flight, and then quit. Postaccident examination of the engine revealed that the number 1 engine cylinder's exhaust valve had failed at the neck of the valve. A Federal Aviation Administration inspector stated that the pilot and the airplane were both properly certificated and in compliance with the regulations at the time of the accident.