On July 10, 1999, at 1230 hours Pacific daylight time, a Piper PA-24-180, N6597P, collided with ground obstructions during a forced landing following a loss of power during the takeoff initial climb at San Luis Obispo, California. The aircraft, owned and operated by the student pilot, sustained substantial damage. The student pilot and his instructor were not injured. The cross-country instructional flight was conducted under 14 CFR Part 91 and a VFR flight plan was filed. The dual cross-country flight originated at Whiteman Airport in Pacoima, California at 1030. Visual meteorological conditions prevailed at the time of the accident. The flight instructor told Safety Board investigators that he met the student pilot at the airplane and watched him preflight the airplane. The airplane was being fueled at the time. He stated he verified the fuel quantity (full left and right main tanks), and fuel cap security after the fueling. The student sumped both tanks with no reported discrepancies. The flight instructor reported that they took off and flew the entire flight to San Luis Obispo on the left tank. They taxied to the ramp area and did not shutdown the engine. He said he had the student change the fuel selector to the right tank at that time, closed their flight plan, and reviewed the return flight. He estimated they were on the ground about 10 minutes, asked for their taxi clearance, and were given taxi instructions to runway 29. The instructor reported that the power came up smoothly on the takeoff roll. He reported that the engine quit approximately 600 feet above ground level (agl). He reported that there was no roughness or surging, and that it just suddenly quit. The flight instructor said he took the controls at that point and lined up on Highway 101. He said he verified that the fuel selector was on the right tank, mixture rich, and the carburetor heat and boost pump was on. He said he had to maneuver the airplane to miss an overpass and proceeded to touchdown in the northbound lanes of the highway. The flight instructor said that the right wing struck a bush, which spun the airplane off the highway and down the embankment next to the highway. At the request of Safety Board investigators, the airplane was removed from the side of the road and taken to an aircraft recovery facility near Pleasant Grove, California. A follow-up examination of the aircraft and engine took place on July 23, 1999, at the recovery facility. The engine had been removed from the airframe and placed on a stand to facilitate the engine examination. Federal Aviation Administration (FAA) inspectors from the Sacramento, California, Flight Standards District Office and a technical representative from Textron Lycoming conducted the subsequent engine examination. According to a report received from the Textron Lycoming investigator, the two bladed constant speed propeller remained attached at the crankshaft flange. The engine data plate identified the subject engine as an O-320-A1A. The top spark plugs were removed and examined. The spark plug electrodes remained undamaged, and, according to the Textron investigator, their coloration was consistent with normal operation. The cylinders were examined at the combustion chamber utilizing a lighted borescope. In the written report, the Textron investigator stated there was no evidence of internal mechanical malfunction or foreign object ingestion. The propeller was turned and piston movement was observed at each cylinder. The Textron investigator noted that during the turning of the propeller, the vacuum pump drive did not turn. The accessory gearbox cover was removed for further examination of the accessory gears. During the removal of the accessory case, the Textron investigator noted that the crankshaft idler gear (left magneto) was detached from the mounting pad at the back of the crankcase. Additionally, he noted that the crankshaft gear was not secure to the crankshaft, and the locating dowel pin was fractured and separated. He did not notice any other damage or mechanical malfunctions. The full flow oil filter was removed and cut open to gain access to the filter media for examination. The filter media contained a large amount of bright silver colored material. Further examination of the idler gear shaft and attaching parts revealed severe wear signatures on the attachment bolts and locking plate. The non-threaded or shank portions of the two 1/4-inch diameter attachment bolts were worn away to approximately 50 percent of their original diameter. The hex head end of the bolts displayed signatures consistent with prolonged contact with the rotating idler gear. Additionally, according Textron's written report, the threaded bolt holes located on the engine case were severely worn and elongated. The bore at the idler gear shaft-mounting pad was "severely worn, elongated and had displaced aluminum material around the circumference." The Textron investigator stated that the wear patterns and material loss were consistent with a condition that "had existed for an extended period of time." He concluded that the two bolt holes of the metal locking plate were elongated and missing a significant amount of material. Further examination of the crankshaft counterbore and corresponding gear revealed fretting and corrosion signatures at the mating surfaces. The dowel pin was separated in line with the parting surfaces of the gear and counterbore. The dowel pin fracture surface displayed visually obvious beach markings. The attachment bolt and locking plate remained intact and undamaged. The locking plate appeared to have been bent against the head of the bolt. The FAA and Textron investigator reviewed the aircraft logbook and located an invoice documenting a field overhaul of the engine in March 1987. The engine had accumulated approximately 1,200 hours of operation since the overhaul. No evidence of a propeller strike or sudden stoppage of the engine was noted during examination of the engine, airframe, and propeller logbooks. It was noted that the current logbooks were begun on August 3, 1988, when the engine had accumulated 400 hours of operation. According to the logbook entries, the original logbooks had been lost. An examination of the propeller logbook revealed that a propeller overhaul had been completed on April 16, 1999, approximately 20 hours prior to the malfunction of the engine. The propeller overhaul facility was subsequently contacted by the FAA inspectors, and it was determined that the propeller was undamaged when the propeller was received. A dynamic propeller balance had been performed after the installation of the propeller. The Chadwick-Helmuth balance card attached to the logbook indicates that 33.6 grams of weight were added to balance the propeller. Safety Board investigators reviewed the standard practices manual section pertaining to static and dynamic balance of the Hartzell propeller. Section C of the manual addresses modification of the spinner bulkhead to accommodate dynamic balance weights. The section states that the placement of balance weights should be outboard of the deice slip ring, or bulkhead doubler, and inboard of the bend at the point the bulkhead creates a flange to attach the spinner dome. Additionally the manual states, "Do not exceed a maximum weight per location of 0.9 oz (25.51 grams) . . . this is approximately equal to six AN970 style washers." The bulkhead on the accident airplane was examined. The FAA and Textron investigator noted that the weights were placed on the outside of the bend at the point the bulkhead created a flange to attach to the spinner dome. Additionally, it was noted that there were seven washers placed in the one location, with a total weight of 36 grams. Textron Lycoming issued Service Instruction SI 1310 in February, 1977, that offered the availability of an improved idler gear shaft and attaching hardware that utilized slotted shear nuts, bolt, and locking wire. Although not required, the examination revealed that the subject installation did not employ the referenced components.