On December 27, 1995, at 1600 central standard time, a Piper PA- 24-250, N5221P, was substantially damaged during a forced landing following a partial loss of engine power near Kaufman, Texas. The private pilot and the passenger received serious injuries. The flight originated from Terrell, Texas, at 1515 and was being operated as a personal flight under Title 14 CFR Part 91. Visual meteorological conditions prevailed and a flight plan was not filed for the local flight. The pilot reported that the engine "started to run rough" and subsequently, as the cockpit filled with smoke a field was selected for a landing. Forward visibility from the cockpit was "limited by the smoke" as the airplane flew downwind to the field. During the final approach the pilot observed a fence across the landing area and "pulled the yoke all the way back." Following the hard landing, the airplane traveled through the fence. The nose gear collapsed, and the engine cowling, fuselage and wings were damaged. Examination of the engine and a review of the maintenance records by the Federal Aviation Administration (FAA) inspector and the manufacturer representative revealed the following information. Engine oil was found on the engine cowling toward the cockpit and in the area of the #4 cylinder. The oil sump contained approximately 1.5 quarts of oil. The exhaust valve push rod and push rod tube on the #4 cylinder were bent and were no longer in place. Crankshaft continuity to the accessory gears was confirmed. Valve action was confirmed on all cylinders except the #4 cylinder. The hydraulic tappet plunger assembly and hydraulic tappet socket for the #4 cylinder were missing. The exhaust valve on the #4 cylinder was "broken off at the push rod end" and "exhibited scoring on the stem." The exhaust valve guide in the #4 cylinder had a "foreign object imbedded in the wall." Engine time since major overhaul was 697.65 hours and the time since the last annual inspection was 26 hours.