On December 15, 1999, approximately 1530 mountain standard time, a Cessna 140, N2739N, sustained substantial damage when it nosed over during landing roll at Silver City airstrip, about 18 miles northwest of Helena, Montana. The private pilot, the sole occupant, was uninjured. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed for the flight, which had originated at Helena, about 1500. The ELT actuated, but was shut off by the pilot immediately after the accident. The pilot stated that on his second touch-and-go landing at the airstrip, about half way down the runway, the left wheel separated and the airplane nosed over. FAA inspectors observed that the left wheel and gear extension had separated from the left main landing gear leg and were found off the runway on the left side. Surface tracks indicated that the airplane had been tracking along the centerline of the runway, and that the airplane had continued to track down the centerline for an additional 30 yards before it nosed over. The four left main landing gear extension attachment bolts were inspected by the NTSB's material laboratory. Examination revealed that all four bolts were bent through the shank portion; however, none of the bolts were separated. Additionally, the threads in each bolt were scraped on the convex side of the bend, as if from sliding out of the bolt hole. Three of the four bolts exhibited what appeared to be stripped threads of their corresponding nuts in their thread grooves. No evidence was found of the presence of a nut on the fourth bolt. See attached materials laboratory factual report for photographs. Records revealed that the airplane had received an annual inspection on November 11, 1999, and it had accumulated seven hours of operation since that inspection.