On September 19, 2014, at 1630 eastern daylight time, a float equipped Cessna 180B, N180GK, was substantially damaged during a forced landing following a total loss of engine power, while maneuvering near Piscataquis County, Maine. The private pilot/owner and passenger sustained minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the flight that departed Mud Pond, Maine about 1530. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.In a telephone conversation with a Federal Aviation Administration (FAA) aviation safety inspector, the pilot said that he was maneuvering about 2,500 feet north of Lake Ross, Maine when he heard a "slight squeal" from the engine before it started to sputter. About 1,500 feet during the subsequent descent, the engine and propeller "completely stopped." The pilot determined the airplane would not reach Lake Ross, and selected an unpaved road in the woods for the forced landing. The right wing struck a tree about 60 feet above the ground, and the airplane impacted the road, resulting in substantial damage to the wings, fuselage, and empennage. The pilot held a private pilot certificate with ratings for airplane single-engine land and sea. His most recent FAA third class medical certificate was issued May 27, 2014. The pilot reported 1,200 total hours of flight experience, of which 950 hours were in the accident airplane make and model. The airplane was manufactured in 1959, and its most recent annual inspection was completed May 24, 2013, at 4,013 total aircraft hours. A Continental O-470K33 remanufactured engine was installed in the airplane on August 20, 2005 and the engine had accrued 605 hours since that date. The wreckage was recovered to a maintenance facility where a cursory examination of the engine was performed. There was no external damage, but a borescope examination of the No. 3 cylinder revealed that the exhaust valve was fractured at the stem and the top of the No. 3 piston was damaged. The engine was retained for further examination. On October 27, 2014, the engine was examined under the supervision of an FAA aviation safety inspector at the manufacturer's facility in Mobile, Alabama. The crankshaft "freely" rotated by hand, and removal of the oil sump revealed "piston material" and what was identified as the No. 3 exhaust valve head, which was fractured at the stem below the head. Disassembly of the engine revealed normal wear and appearance on all cylinders with the exception of the No. 3 cylinder, piston, and exhaust valve. The magnetos were tested, and both produced spark at all terminal leads. The fractured exhaust valve head, stem, and rotocoil were submitted to the manufacturer's engineering department for metallurgical examination. The report prepared by the manufacturer stated that the valve fractured at the transition of the stem to the valve, also exhibited post separation damage. The fracture initiated at the outer surface of the valve stem and grew in fatigue. An NTSB metallurgist reviewed the reports generated by these exams, and concurred with their findings.