On September 28, 2017, at 1716 central daylight time, a Piper PA-25-235 airplane, N7336Z, impacted terrain following a loss of engine power while maneuvering at low altitude near Westphalia, Kansas. The commercial pilot, who was the sole occupant, was not injured, and the airplane sustained substantial damage. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed, and no flight plan had been filed for the flight. The local flight departed the Garnett Municipal Airport (K68), Garnett, Kansas, about 1630. According to the pilot, the airplane departed K68 with full fuel and chemical for the aerial application flight. After spraying a field for about 20 minutes, the engine lost power. The pilot jettisoned his chemical load and initiated a forced landing to a field. About 5 feet above the field as the pilot tried to bleed off airspeed, the airplane stalled and impacted terrain. The airplane sustained substantial damage to the left wing. The Lycoming O-540-B2B5 engine was removed from the airframe for further examination. A review of the maintenance records showed the engine underwent a field major overhaul on January 28, 2016, about 218 hours prior to the accident. On October 25, 2017, the engine was disassembled under the supervision of the Federal Aviation Administration inspectors. Disassembly of the engine revealed the crankshaft was fractured, and the main bearings displayed thermal damage and smearing. The crankshaft and main bearings were retained for further examination. The fractured crankshaft and forward main journal bearings were examined by the National Transportation Safety Board Materials Laboratory. Examination of the crankshaft showed a fracture across a web located between the second cylinder (C2) journal and the second main (M2) journal. Approximately 60% of the fracture surface exhibited a general flat portion with crack arrest marks. The crack arrest marks appeared to emanate out from near the corner of the web near the M2 journal. The crack arrest features were not consistent with having progressed from a surface or from an internal feature, but were consistent with a subsurface crack initiation, and fatigue crack propagation. The fatigue striations, particularly near the initiation site, were consistent with high-cycle fatigue. The forward main bearing (M1) journal displayed discoloration and dark circumferential wear marks on the aft most two inches. One aft bearing surface revealed spalling, displacement, and cracking consistent with bearing wear fatigue. The other aft bearing surface exhibited spalling, material displacement, and smearing. Much of the spalled and smeared material was found accumulated in the lubrication groove. All the bearing damage was typical of similar bearings that operated at elevated temperatures with inadequate lubrication.