On November 26, 2005, about 1153 eastern standard time, a Piper PA-24-180, N6858P, was substantially damaged when it impacted trees and terrain during departure from Lincoln Park Airport (N07), Lincoln Park, New Jersey. The certificated private pilot and passenger were seriously injured. Visual meteorological conditions prevailed, and no flight plan was filed for the flight, destined for Sky Manor Airport (N40), Pittstown, New Jersey. The personal flight was conducted under 14 CFR Part 91. According to the pilot, everything was normal until the airplane became airborne. As it passed over a row of trees at the edge of the airport, the engine "began to feel off balance" and began "misfiring". Since the engine continued to run at partial power, the pilot raised the flaps and began a left turn to return to the airport. A few moments later, the pilot heard the engine begin to "backfire," and it lost all power. The airplane impacted trees, rotated 180 degrees to the left and came to rest in a small clearing. Examination of the engine and propeller revealed that the propeller blades exhibited no leading edge gouging or chordwise scratching of the blade faces. Fuel was observed in the carburetor, engine driven fuel pump, and electric boost pump. Spark was obtained at all harness leads from both the left and right magnetos. Valve train and crankshaft continuity was confirmed. Thumb compression was obtained at all cylinders, with the exception of the number 3 cylinder. External examination of the number 3 cylinder revealed that a helicoil appeared to be cross threaded in the cylinder head. A compression test of the number 3 cylinder was performed utilizing an approximate 50-psi input pressure; however, the cylinder would not maintain pressure and was leaking past the intake valve head and valve seat area. Internal examination of the number 3 cylinder revealed that the inside of the intake passage was discolored. The intake valve would not close or seat due to material that was adhering to the valve and valve seat. It was discovered that the material was metallic in nature, non-magnetic, and was also adhering to the cylinder head and top of the piston. On November 13, 2005, the pilot contacted a maintenance provider and advised that he was having trouble "clearing one of his spark plugs." He removed the number 3 cylinder bottom spark plug, and asked the mechanic if he could "look at the plug." The mechanic reported that the bottom half of the plug thread was missing. He noticed heavy deposits of rust on the bottom flange of the sparkplug and that the cylinder head threads were in poor condition. After removing the old threads, he inserted a magnet through the top sparkplug hole to "make sure there were no steel particles." He then cleaned the hole, inserted a new helicoil, installed a new sparkplug, and "ran up" the engine. According to maintenance records, the helicoil installation was inspected by a certificated airframe and powerplant mechanic who held an inspection authorization. According to the non-certificated mechanic, at the time the work was performed, the certificated mechanic was not present. According to Textron Lycoming Service Instruction No. 1043A, if a spark plug Helicoil insert replacement is performed with the cylinder assembled on the engine, "some provision must be made for preventing the aluminum shavings from entering the combustion chamber." The pilot held a private pilot certificate with ratings for airplane single-engine-land, and airplane single-engine sea. He reported a total flight time of 2,220 flight hours, with 1,874 hours in the accident airplane make and model. His most recent FAA third-class medical certificate was issued on May 26, 2004. A weather observation taken at the time of the accident at the Essex County Airport (CDW), Caldwell, New Jersey, located approximately 4 nautical miles south of the accident site, included winds from 190 degrees at 6 knots, visibility 10 miles, clear skies, temperature 36 degrees Fahrenheit, dew point 14 degrees Fahrenheit, and an altimeter setting of 30.35 inches of mercury.