On November 21, 2004, at 1430 central standard time, an amateur-built Olson Sonex, N478SX, built and piloted by a private pilot, was substantially damaged when it nosed-over during a forced landing following an in-flight separation of the propeller near Oconomowoc, Wisconsin. Visual meteorological conditions prevailed at the time of the accident. The personal flight was operating under the provisions of 14 CFR Part 91 without a fight plan. The pilot sustained serious injuries. The local flight departed Watertown Municipal Airport (RYV), Watertown, Wisconsin, at 1400. The pilot reported that during cruise flight the engine developed a sustained vibration and he elected to return to RYN. The pilot reported that the "engine stopped abruptly" and he decided to perform a forced landing on a road being constructed near Oconomowoc. He stated that the airplane was "Too low to clear bridge ... and [he] diverted to open cornfield." The pilot reported that the airplane's main landing gear impacted a "small ditch" during landing and the airplane subsequently nosed-over. Additionally, the pilot stated that the engine had a vibration during the previous flight and he re-torqued the propeller to the hub after the flight. A Federal Aviation Administration (FAA) inspector performed an on-scene investigation. The inspector reported the airplane was located in a cornfield in an inverted orientation. The propeller and hub assembly were not located at the accident site and were not recovered during the investigation. The aircraft was transferred to a local law enforcement impound lot for further inspection. The FAA inspector successfully started and ran the engine at various power settings. The engine was then removed from the airframe and transported to the engine kit manufacturer in Oshkosh, Wisconsin, for further examination. The engine was a modified Volkswagen 2180cc series engine that produces 80 horsepower. According to the engine kit manufacturer, Aero-Conversions, the engine components provided are all factory-new and have been modified for aircraft use. The "Aero Vee 2002" engine incorporates a counterweighted forged crankshaft and a shrink-fit propeller hub. The engine kit is shipped unassembled to the purchaser. The forward portion of the crankshaft contains the number two bearing, camshaft gear (p/n 1925), racer spacer (p/n 1919), distributor drive gear (p/n 1927), retaining snap-ring (p/n 1928), number one bearing, oil-slinger (p/n 1929), and propeller hub (p/n ACV-H01-10). The camshaft gear, racer spacer and distributor drive gear are indexed by a Woodruff key (p/n 1937) and held in place by the retaining snap-ring. The propeller hub is indexed by a rounded Woodruff key (p/n 1938) and is retained by a bolt (p/n 38458). The camshaft gear, racer spacer, distributor drive gear and propeller hub are designed as shrink-fit components. Prior to assembly, the shrink-fit components are heated in an oven to 400-degrees for 30 minutes. After the components are installed and fully cooled, engine torque is transferred to the components through friction forces and the two Woodruff keys. The designed interference for the propeller hub is between 0.0010 and 0.0015 inches, according to the engine kit manufacturer. The heated propeller hub is positioned on the crankshaft by striking a block of wood with a mallet against the forward face of the hub. Once the hub is properly positioned against the oil-slinger a pneumatic wrench is used to temporally install the hub bolt. After the hub has completely cooled, the hub bolt is removed and reinstalled with Loc-Tight and torqued to 80 ft/lbs. FAA inspectors supervised a teardown inspection of the engine on November 24, 2004. The engine crankshaft was fractured between the oil-slinger and the retaining snap-ring groves. The fracture initiated at the aft edge of the forward keyway. The fracture features were consistent with a fatigue-type failure. The camshaft gear, racer spacer and distributor drive gears were found loose on the crankshaft and the assembly had a 0.032 inch gap between the retaining snap-ring. The design gap tolerance for the assembly is between 0.002 and 0.005 inch. The Woodruff key (p/n 1937) exhibited damage consistent with excessive loading, according to engine kit manufacturer. The pilot reported that he originally had difficulty shimming the aft-mounted flywheel after assembly of the engine. He stated he had mistakenly installed the number four (aft-most) flanged bearing at the number two position during initial assembly of the engine. The pilot had to remove the shrink-fit propeller hub, spacer and gears in order to reinstall the bearings in the correct order. The pilot reported he heated the shrink-fit components with a propane torch and removed the propeller hub using a homebuilt puller. The puller consisted of a six-inch diameter machined aluminum disk and a jackscrew. The pilot/builder stated that he did not check the dimensions of the shrink-fit components prior to reassembling the engine, nor did he consult with the engine kit manufacturer on how to disassemble, inspect, and reassemble the engine components. The FAA issued an experimental airworthiness certificate for the amateur-built airplane on July 7, 2004. The airframe and engine do not have to comply with federal regulations required for a standard category airplane. The airplane and engine had accumulated 45.3 hours since new. The engine kit manufacturer did not provide the builder with engine disassembly or overhaul instructions for the "Aero Vee 2002" engine. The manufacturer also did not instruct builders to verify the dimensions of the shrink-fit components prior to their installation. However, as result of the accident investigation, the manufacturer stated that it planned to provide documentation for the disassembly of the engine and how to verify the dimensions of the shrink-fit components prior to assembly.