On May 27, 2017, about 1330 eastern daylight time, a privately operated Avions Mudry CIE CAP 231, N231X, was substantially damaged during a forced landing at the Keller Brothers Airport (08N), Lebanon, Pennsylvania. The private pilot was not injured. The airplane was being operated under the provisions of 14 Code of Federal Regulations Part 91as a personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight which originated about 10 minutes earlier from 08N.The pilot stated that he was over 08N in the active aerobatics box practicing maneuvers, and had completed a hammerhead, a Cuban 8, another hammerhead, and two verticals. He was in a 45° nose-up climb maneuver when he heard a loud knock from the engine followed by a significant "shake." He leveled the airplane, reduced manifold pressure and announced that he was returning to 08N over the common traffic advisory frequency. As he turned towards the runway, the engine rattled and shook more, and then stopped producing power. The pilot attempted to restart the engine but was unsuccessful. He planned a high base leg and side slip onto the final approach leg of the airport traffic pattern to assure that the airplane would reach the runway. While gliding, he determined that the airplane felt unstable under 100 mph, and conducted the approach at a speed of 100 mph. During the subsequent landing flare, the airplane floated, touched down beyond midpoint of the nearly 2,700 ft long grass runway, and bounced. The pilot applied hard braking; however, the airplane rolled off the end of the runway, down an embankment, and nosed over. Examination of the AEIO-540-L1B5D engine following recovery revealed the engine was seized, and there was a hole in the crankcase near the crankcase parting line below the intake and exhaust valve pushrods. The No. 4 cylinder connecting rod cap was fractured, a nut from the No. 4 connecting rod was recovered from inside the crankcase, and the No. 4 exhaust valve cam follower was found inside the lower engine cowling. Considerable force was required to break the crankshaft free. Disassembly examination of the engine with FAA oversight revealed that one counterweight, which was normally secured to the crankshaft at two places, was separated at one location. The retaining ring, washer, and roller associated with the separated side of the counterweight were separated. The separated washer and retaining ring were fractured and were found in the oil sump, while the roller was jammed between the crankshaft and engine crankcase. The separated roller and pieces of the fractured retaining ring and washer were retained. The remaining counterweight roller retaining rings were properly installed in the counterweights. Additional retained components consisted of the other counterweight, a remnant of a connecting rod bearing, a fractured connecting rod cap with fractured section of connecting rod bolt, mating section of fractured connecting rod bolt without a nut, a nut associated with a connecting rod that was found from the top of the engine, and a nut associated with a connecting rod that was found inside the engine. The fractured components were forwarded to the NTSB Materials Laboratory for detailed examination.. Examination of the fractured pieces of the retaining ring associated with the counterweight revealed the fracture surfaces were consistent with overstress fracture, and the surfaces were consistent with the fractured retaining ring still present in the counterweight assembly. The general circular fracture features were consistent with rotational loading. The fracture surface of one piece of the fractured washer exhibited progressive features (such as crack arrest marks and radial patterns), consistent with reverse bending fatigue, with crack initiation on the outside faces of the washer. However, the fatigue crack initiation sites were destroyed by post-fracture damage. The outer surface of the roller exhibited both axial and radial sliding wear. No other pre-existing fracture was noted on any of the other submitted components. Examination of all circlips revealed evidence they were properly installed. Review of the maintenance records revealed the engine was last overhauled on November 5, 2009. As part of the overhaul, new counterweight bushings, washers, and retaining rings were installed. The engine had accumulated about 242 hours since overhaul at the time of the accident. According to a representative of the engine manufacturer, when operational conditions or non-standard movement cause the rollers to slam against the counterweight washers, impact bending forces on the counterweight washers can and does occur, sometimes causing fatigue cracks to initiate on the outer face of the washers (typically at the corners). The representative indicated that operational considerations for non-standard movement and impact loading by the rollers against the counterweight bushings and/or counterweight washers include: 1. Sudden stoppage, such as prop strike etc.; 2) Sudden accelerations and decelerations as in rapid throttling; 3) Counterweight Detuning; 4) Operating at excessive power/overloading; 5) Operating under detonation conditions; 6) Abnormally high torsional vibrations, and; 7) Improper installation/assembly (wrong part numbers of rollers, or miss-matched rollers in the counterweights).