On June 02, 2021, about 1000 Pacific daylight time, a Cessna 172G, N3871L, was substantially damaged when it was involved in an accident near Washington, California. The pilot and the passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight. The pilot reported that the airplane was climbing at full power at 80 mph, and he progressively leaned the mixture during the climb. About 30 minutes into the climb profile, the airplane’s altitude was near 6,000 ft msl, and the pilot observed that the airplane’s engine oil temperature gauge had reached its maximum limit of 240°F. He attempted to arrest the increasing oil temperature by lowering the nose of the airplane to increase airspeed and subsequently decreased the engine power setting. About 10 minutes later, the oil temperature decreased below 240°F and the pilot resumed his climb to 9,500 ft msl using a richer mixture setting. About 8,500 ft msl, the pilot heard the engine produce a “backfire” type sound, and confirmed a loss of engine power as he observed the engine rpm gradually decrease. The pilot began to troubleshoot but was unable to restore engine power. Unable to maintain altitude, he declared an emergency and maneuvered the airplane toward a clear area within the mountainous terrain. During the forced landing, the airplane sustained substantial damage to both wings and the lower fuselage. A review of the airframe and engine maintenance logbooks indicated that the last annual inspection was completed 6 months before the accident, on December 6, 2020. At the time of the annual inspection, the engine had amassed a total time of 4,027 hours, and 1,896 hours since major overhaul. The engine tachometer indicated 2,277 hours during the last annual inspection and indicated 2,288.8 at the time of the accident. The engine-manufacturer recommended overhaul interval is 1,800 hours or 12 years, whichever occurs first. Examination of the engine revealed that magneto-to-engine timing was Left: 33° before top dead center (BTDC) and Right: 32° BTDC. The engine manufacturer specification for magneto-to-engine timing was Left: 28° BTDC and Right: 26° BTDC. About 6 qts of engine oil were extracted from the oil sump. The No. 2 cylinder displayed low compression. All cylinders were removed from the crankcase for further examination. Removal of cylinder Nos. 1 and 4 revealed no anomalies. Removal of the No. 2 cylinder revealed that the top compression ring was seized to the piston ring land, with flat spots observed on the top ring. Removal of the No. 3 cylinder revealed that the top compression ring was broken into two pieces, and the exhaust valve was stuck in the valve guide. Removal of the No. 5 cylinder revealed excessive key-holing of the intake valve within its guide. The No. 6 cylinder top compression ring was broken into four pieces.