On September 19, 2015, about 1200 eastern daylight time, a Cessna 150M, N714CW, was substantially damaged following a precautionary landing in a field near Mount Airy, Maryland. The private pilot was not injured. The airplane was operated under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed about the time of the accident and a visual flight rules (VFR) flight plan was filed for the flight that departed Clearview Airpark (2W2), Westminster, Maryland at 1130 and was destined for Leesburg Executive Airport (JYO), Leesburg, Virginia. The pilot reported that 10 minutes after takeoff, at an altitude of 2,500 feet, the engine was not making full power. The pilot monitored the situation and adjusted his path to put him over Davis Airport (W50), Laytonsville, Maryland, in the event he needed to land. He continued to monitor the situation and noted that the engine "was a little sluggish," and became progressively worse, then it started to run rough before losing more power. All engine instrument indications were normal. He declared an emergency and observed a field to land on. As he circled to land, he saw electrical wires on the approach end, so he flew downwind for a landing in the opposite direction. During the turn to final, there was not enough altitude to make the landing site, so he landed on an adjacent field planted with mature corn. As the airplane settled into the corn, it touched down, then flipped over. According to Federal Aviation Administration (FAA) airworthiness and airplane maintenance records, it was powered by a Continental O-200-A, 100-horsepower engine, driving a McCauley metal two-blade fixed pitch propeller. The pilot reported the engine had 3,567 hours total time and 1,800 hours total time since overhaul. The airplane accrued 15.7 hours since the last annual inspection on October 23, 2014. According to the Major Repair and Authorization form OMB No. 2120-0020, the engine had 4 Superior-SA10200 series cylinders installed on May 10, 1995 during the last engine overhaul. Carroll County Regional Airport/Jack B Poage Field (DMW) Westminster, Maryland was located about 8 miles north of the accident site. The DMW recorded weather at 1230 included wind from 200° at 9 kts gusting to 14 kts, visibility 10 statute miles, clear sky, temperature 27°C, dewpoint 17 ° C, and the altimeter setting was 29.96 inches of mercury. An inspector with the FAA responded to the accident site and examined the wreckage. The airplane and all major components were accounted for at the scene. There was substantial damage to the underside of the fuselage where the nose landing gear was sheared off and damage to the firewall and engine mounts. Both wing spars were damaged and the vertical stabilizer was bent. In addition, there was damage to the no. 4 cylinder and oil streaks were observed on the underside of the engine cowling and fuselage. The propeller was intact and undamaged. Subsequent detailed examination of the engine revealed that the no. 4 cylinder (part # SA10200) fractured at the barrel portion about 2 inches out from the engine case, completely separating from the bottom portion of the cylinder that remained attached to the case. The no. 4 piston was gouged and missing a large amount of material. There was a substantial amount of oil and fragments of metal discovered in the engine compartment. The no. 4 cylinder, piston and wrist pin were removed and sent to the NTSB Materials Laboratory for additional examination. A microscopic examination of the upper fracture face revealed a dark thumbnail mark consistent with a fatigue crack that emanated from the outer surface of the barrel. The fatigue crack originated from a corrosion pit and propagated through the thickness of the barrel and extended about 2 ½ inches to each side of the fatigue origin area. The piston exhibited a rough texture on a slant plane typical of overstress separation.