On October 3, 1996, at 1215 hours Pacific daylight time, a Piper PA-28-140, N5705U, made a forced landing in a vineyard in Plymouth, California, following a loss of engine power. The aircraft sustained substantial damage; however, neither the pilot nor his two passengers were injured. The aircraft was being operated as a personal flight by the pilot/owner when the accident occurred. The flight originated in Jackson, California, at 1140 on the morning of the accident and was en route to Auburn, California. Visual meteorological conditions prevailed at the time and no flight plan was filed. The pilot reported that he was in cruise at 5,000 feet msl when he experienced severe engine vibrations. When he checked his engine instruments he noticed a drop in engine rpm, EGT, and engine oil pressure, along with a loss of power. The engine rpm and oil pressure continued to drop and he was unable to maintain altitude. About the same time he reported smoke in the cockpit, and subsequently executed a forced landing in the driveway of the Shenandoah Vineyard. During the landing the aircraft's right wing struck a fence post and separated 1.5 feet from the outboard tip. On October 11, 1996, an FAA airworthiness inspector and representatives of the aircraft and engine manufacturers conducted a postaccident inspection of the aircraft. Examination of the No. 3 spark plug revealed impact damage on the electrode section. A thumb compression check revealed no compression at the No. 3 cylinder. Further investigation revealed that the No. 3 cylinder had ingested portions of the No. 3 exhaust valve. The valve head was separated from the stem and had penetrated the piston face. The exhaust valve fracture surfaces were peened. Further examination revealed an induction leak at the induction pipe flange gasket of the No. 3 cylinder. The exhaust pipe of the No. 3 cylinder was also detached. One of the attaching studs was found in the bottom of the cowling with its threads intact and undamaged. The exhaust port flange exhibited erosion. The FAA airworthiness inspector reported that the aircraft had recently been purchased by the current owner and undergone an inspection on August 21, 1996, at Rialto Aircraft Services. Since there was no corresponding entry in the aircraft logbooks, the inspection invoice was reviewed by the inspector. The invoice identified exhaust port erosion on the No. 3 cylinder. No corrective action was found in the aircraft and engine maintenance logs for any of the discrepancies. The owner reported that the engine was close to TBO. The inspector, who reviewed the logbooks and service invoice, concluded that the aircraft had been in an airworthy condition prior to the accident.