On June 18, 2011, about 1635 central daylight time, a Cessna 172N, N172NT, was substantially damaged when it nosed over during a forced landing following a loss of engine power about 2 miles southwest of Midway Airport (MDW), Chicago, Illinois. The pilot and 3 passengers were not injured. The aircraft was registered to Woma LLC and operated by Aviation Professionals Inc. under the provisions of 14 Code of Federal Regulations Part 91, as a training and rental airplane. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local personal flight originated from Lansing Municipal Airport (IGQ), Lansing, Illinois, about 1610. The pilot reported that about 5 miles from MDW the engine started to run rough, and about 20 seconds later the engine lost power. His attempts to restore engine power were unsuccessful. The pilot subsequently executed a forced landing to a field. However, the airplane nosed over during the landing rollout due to the soft terrain. He noted that the field was used as a water retention reservoir with 2-foot tall grass and approximately 4 inches of water and mud. A postaccident examination revealed that the #3 cylinder exhaust valve push rod had buckled and fractured near mid-length. The fracture surface was uniform in appearance and oriented approximately 45-degrees to the surface of the push rod consistent with an overload failure. At the time of the examination, the exhaust valve was free to move. However, the damage to the push rod was consistent with a stuck valve condition during operation. No other engine anomalies were observed. The accident airplane was powered by a 160-horsepower Lycoming O-320-H2AD engine, serial number L-80290-76T. Maintenance records indicated that the most recent annual inspection was completed on May 10, 2011. The engine had accumulated about 1,567 hours since overhaul, with approximately 67 hours since the annual inspection.