On September 3, 2000, at 1840 hours Pacific daylight time, a Piper PA-28-161, N6462C, was substantially damaged when the left main landing gear collapsed during landing at Palo Alto Airport of Santa Clara County, Palo Alto, California. The private pilot and one passenger were not injured. The personal flight, operated under 14 CFR Part 91 by ZP Aviation, Inc., departed from Monterey, California about 0500. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he landed the rental aircraft "somewhat hard" and collapsed the left main landing gear of the fixed gear airplane. According to the pilot, subsequent inspection of the aircraft revealed pre-existing damage to the bolts that attach the main landing gear to the spar. He reported that the landing gear casting is bolted to the aft side of the spar with 8 bolts; 4 in a rectangular pattern on the top, and 4 in a similar pattern on the bottom. Of the bottom 4 bolts, 2 were absent, the bolt head was missing on the third, and the fourth was loose. All 4 of the upper attach bolts were loose. A mechanic, who inspected the aircraft after the accident, confirmed that the two missing bolts were not found in the wing. He added that there was fretting at the casting-to-spar interface, and that the attachment holes were elongated. He also added that the 8 bolts on the right main landing gear attachment, which did not fail, were all loose. According to an investigator at The New Piper Aircraft Company, there are no Airworthiness Directives or Service Bulletins in effect regarding the landing gear to wing spar attachment on this model aircraft. The Piper Airplane Maintenance Manual for the PA-28-161 airplane, dated February 28, 1995, contains an Inspection Checklist for the 100-hour inspection. In the landing gear group of the checklist, item 16 states to "Inspect gear struts, attachments, torque links and bolts for condition and security."