On September 3, 1999, about 1751 eastern daylight time, a Piper PA-18-135, N8143E, was substantially damaged during a forced landing after takeoff from the Aeroflex-Andover Airport (12N), Andover, New Jersey. The certificated flight instructor (CFI) and passenger received minor injuries. Visual meteorological conditions prevailed and no flight plan was filed for the instructional flight conducted under 14 CFR Part 91. The CFI departed 12N about 1700, and remained in the local area for an instructional flight. Approximately 20 minutes later he returned to the airport, landed, and kept the engine running while the student exited the airplane. A passenger was then boarded for an orientation flight. The CFI stated that he departed Runway 21, and about 400 feet above the ground, the engine made "a metallic noise." He described this sound as "similar to a valve tap, only much louder." The CFI lowered the airplane's nose and initiated a 180 degree left turn, to land on Runway 03. A few seconds later, the engine powered down and stopped, with the propeller in a horizontal position. The turn was completed, and as the airplane approached the runway, the main wheels struck the bank of a pond approximately 100 feet short of the runway, the airplane nosed over, and the pilot and passenger exited the aircraft. The CFI observed fuel leaking from the aircraft and a small fire under the engine cowl. He extinguished the fire with dirt, flipped the airplane upright to stop fuel from leaking, closed the mixture, and turned the fuel and the magnetos to the off position. An examination of the engine on September 7, 1999, revealed that the crankshaft had separated at the aft edge of the #3 connecting rod. The camshaft was bent outward and a dent was observed adjacent to the break on the crankshaft. Metal fragments and a portion of a valve lifter were found in the oil pan. Metal fragments were also found in the oil finger screen. All four cylinders and their pistons displayed no visible damage. The crankshaft ring gear bolt was sheared and its mounting flange was bent. According to the Safety Board Materials Laboratory Factual Report, the crankshaft was separated through the aft cheek of the #3 main bearing. Magnified visual examination of the fracture face found characteristics of fatigue propagation, which initiated in the aft radius of the #3 main bearing and progressed rearward through approximately 80% of the crankshaft cheek before failing in overload. The report also stated that examination of the initiation area revealed a single origin on the surface of the radius in an area of circumferential scratches. The origin was located on the journal about 10 to 15 degrees before top center of the adjacent #2 and #3 cylinders. Both the forward and aft radii of the #3 main journal contained circumferential patterns of scratches and scuff marks consistent with contact by the bearing shells. In addition, both radii of the #3 main journal appeared clean and did not exhibit the tarnish pattern found on the other journal radii of this crankshaft. All of the journal surfaces showed a light circumferential scratch pattern; however, measurements of all of the journal surfaces revealed that the #3 journal was 0.001 to 0.002 inches smaller in diameter than the other main journals. Examination of the airplane's logbook revealed that on February 20, 1999, a 100 hour inspection was performed. The entry which described the inspection also stated, "...removed Sensenich propeller and installed a new McCauley propeller." On July 23, 1999, an entry was made into the logbook which stated "...removed McCauley propeller for overhaul due to large nick in blade, installed Sensenich propeller...". The last entry made into the airplane's logbook was on September 1, 1999, which stated that a 100 hour inspection was performed and the "airplane was determined to be in an airworthy condition." In a telephone interview, the mechanic who performed maintenance on the accident airplane stated that the McCauley propeller was installed on the airplane because it was a "high performance propeller" and the operator hoped to "get more power out of the airplane." The mechanic reported that the McCauley propeller was removed from the airplane and the Sensenich propeller was reinstalled when the operator determined that the McCauley propeller was not producing "that much more power," and it had a "small nick" on one of the blades. According to the mechanic, the "small nick" on one of the propeller blades resulted when the airplane "picked up a rock, possibly during a short field takeoff."