HISTORY OF FLIGHTOn November 25, 2012, about 1505 Pacific standard time, a Cessna U206, N5229U, collided with a fence during an off field forced landing near Van Nuys, California. The pilot/owner was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot and one passenger sustained minor injuries; the airplane sustained substantial damage from impact forces. The cross-country personal flight departed Gnoss Field Airport, Novato, California, with a planned destination of John Wayne-Orange County Airport, Santa Ana, California. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot stated the airplane was in cruise flight when the engine started backfiring. He smelled burnt oil, and the engine subsequently lost power. During the forced landing on a soccer field, the airplane collided with a fence and trees before coming to rest on its left side. AIRCRAFT INFORMATIONThe airplane was a Cessna 206, serial number 206-0229. The operator reported that the airplane had a total airframe time of 8,525 hours at the last annual inspection. It had an annual inspection dated October 4, 2012. The tachometer read 2,850 at the last inspection. The engine was a Continental Motors, Inc. (CMI), IO-520-A, serial number 112894, assembled on April 26, 2000. At the time of the last inspection, the engine had accumulated a total of 1,054 hours. AIRPORT INFORMATIONThe airplane was a Cessna 206, serial number 206-0229. The operator reported that the airplane had a total airframe time of 8,525 hours at the last annual inspection. It had an annual inspection dated October 4, 2012. The tachometer read 2,850 at the last inspection. The engine was a Continental Motors, Inc. (CMI), IO-520-A, serial number 112894, assembled on April 26, 2000. At the time of the last inspection, the engine had accumulated a total of 1,054 hours. TESTS AND RESEARCHJanuary 15, 2013 Examination The National Transportation Safety Board investigator-in-charge (IIC), and investigators from Cessna and CMI examined the wreckage at a hangar in Santa Ana, California, on January 15, 2013. A full report is contained within the public docket for this accident. Investigators were unable to manually rotate the engine. At the time of the investigative exam, the tachometer read 2,878.3 hours (28.3 hours after the annual inspection). Examination of the engine revealed that the exhaust pushrod housing tubes on the number two and number five cylinders were fractured through their entire circumference. The exhaust pushrod shroud on the number three cylinder was cracked approximately halfway around its circumference. All three cracks/fractures were located along the pressed bead that serves as a seat for the pushrod housing spring. Investigators crated the engine for shipment to the factory for further examination. April 8, 2013 Examination - Mobile, Alabama CMI personnel examined the engine under the supervision of the IIC at the factory in Mobile, Alabama, on April 8, 2013. A full report is contained within the public docket for this accident. A borescope inspection revealed that a valve in the number two cylinder was in contact with a piston face. All piston heads exhibited semicircular indentations near the location of the valves. The camshaft, which operated the cylinder valves, was driven by a gear that meshed with a gear on the crankshaft. Upon disassembly, six gear teeth on the crankshaft gear were fractured. All teeth on the camshaft gear were ground off. A CMI metallurgist examined several of the separated crankshaft gear teeth. Two fracture surfaces showed fatigue with areas of corrosion pitting. The others exhibited overload.