Accident Overview
The pilot executed a forced landing into rough terrain after hearing a bang and experiencing a sudden loss of engine power. The engine had accumulated approximately 70 hours since its last overhaul.
Investigation Findings
Upon disassembly, the engine sump contained a connecting rod, a wrist pin, bearings, metal debris, and several rod cap bolts and nuts. None of the rod cap bolts found in the sump were intact; all were bent, fractured, and separated approximately at their midpoint. The connecting rods for cylinders number two and four were leaning on the camshaft. The camshaft was not discolored, and the crankshaft showed no evidence of discoloration or mechanical damage.
Metallurgical Analysis
A metallurgical examination of various engine pieces and fracture surfaces strongly suggested that the nuts were not tightened on the connecting rod bolts according to specifications. A bolt corresponding to the head of the bolt from cylinder number two exhibited failure characteristics of low cycle-high stress reverse bending fatigue, which is indicative of an under-torqued bolt. Additional evidence included flattened and polished bolt threads, wear marks on bolt outer diameters, and nut threads with only one end flattened. No fractured bolt had a nut attached except the fatigued bolt.