On December 18, 2002, at 1521 Pacific standard time, a Schweizer 269C, N642KC, experienced a total loss of engine power and rolled over during a forced landing in a muddy field at Los Banos, California. The helicopter was operated by the pilot under 14 CFR Part 91. The commercial pilot and one passenger were not injured. The helicopter was substantially damaged. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated at Los Banos about 1500. The pilot and aircraft maintenance technician were performing a post maintenance test flight when a vibration occurred followed by a total loss of engine power. Post accident examination revealed a failed exhaust valve in the number 3 cylinder. Further examination revealed two exhaust spring seats had been installed, one on top of the other, causing excessive spring/valve pressures. The operator had just overhauled the engine; the failure occurred about 1.5 hours after the overhaul.