On February 18, 2001, at 1430 central standard time, a Piper PA-22-108, N5886Z, operated by a commercial pilot, sustained substantial damage when the airplane nosed over during the landing roll following a precautionary landing on a cornfield. The accident occurred approximately 25 miles west of Kearney, Nebraska. Prior to the accident, the pilot said that the airplane's engine was running rough. Visual meteorological conditions prevailed at the time of the accident. The personal flight was being conducted under the provisions of 14 CFR Part 91 without a flight plan. The pilot reported no injuries. The cross-country flight originated at Kearney, Nebraska, at 1415, and was en route to Cheyenne, Wyoming. In his written statement, the pilot said that just south of Overton, Nebraska, the airplane's engine started running rough. The pilot said he elected to land in a corn stubble field. "I touched down normally and on roll out the aircraft contacted a snow drift and went over on its back." A Federal Aviation Administration inspector examined the airplane at the accident site. The airplane was found resting inverted in a cornfield. Both of the airplane's wings were bent downward. The vertical stabilizer and rudder were crushed downward. The windscreen was broken out. The smell of fuel was prevalent. Flight control continuity was confirmed. An examination of the airplane's engine revealed that the number 4 cylinder exhaust valve guide was sticking and the exhaust valve was not seating properly.