Overview
During a flight at 5,500 feet MSL, the pilot observed a loss of oil pressure followed by a loss of engine power. The pilot executed a forced landing in an open field, during which the airplane collided with a ditch. Post-accident inspection of the airplane revealed that the lower fuselage was covered with oil.
Engine Examination
Examination of the engine disclosed evidence of oil starvation in the main bearings, connecting rod bearings, and crankshaft journals. All cylinders were steel-barreled except the #4 cylinder, which was chrome plated. The #4 cylinder exhibited heavy scoring in the chrome barrel, and the #4 piston was scored completely around its skirt. The lower crown of the #4 piston showed burning that had eroded a hole into the piston interior below the third piston ring groove. The piston rings were stuck due to aluminum from the piston skirts extruding over them. Heat had expanded the piston skirts into contact with the cylinder walls, causing scoring and transfer of aluminum from the piston skirt.
Manufacturer's Report
According to a report from the engine manufacturer, chrome cylinders must use plain steel rings per the manufacturer's service literature. However, the #4 cylinder had been installed using rings intended for a steel-barreled cylinder, which feature chrome plating on the top compression ring. The manufacturer's report noted that a chrome ring on a chrome cylinder bore is incompatible and could cause scoring, metal transfer, and eventually severe blowby conditions. The engine had accumulated 1,075 hours of flight time since its last overhaul.
Outcome
The forced landing resulted in damage to the airplane from the collision with the ditch. The engine failure was attributed to oil starvation and subsequent mechanical damage stemming from the improper ring installation.