On January 20, 2011, about 1317 Pacific standard time, a Piper PA-28-235, N8608W, sustained substantial damage during a forced landing near the San Luis County Regional Airport (SBP), San Luis Obispo, California. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. The private pilot, sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight. The local flight originated from SBP at 1130. The pilot reported that while on approach to runway 29, he noticed that the vacuum pump failed. The pilot continued flying towards SBP, and was asked by air traffic control to slow the airplane for traffic ahead of his position. The pilot said that he reduced power to comply with the controller's request. Shortly after, the pilot advanced the throttle to arrest the descent, and noticed the engine did not respond. The pilot said that he realized the engine had lost power, and that he would not be able to make the airport, so he initiated a forced landing to a field adjacent to his position. During the forced landing, the airplane impacted multiple metal poles within a vineyard and came to rest upright. Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the left and right wings sustained structural damage. All four fuel tanks (auxiliary and main) were breeched. The odor of fuel was present at the accident site after the time of the accident. The airplane was recovered to a secure location for further examination. Examination of the recovered engine revealed that it remained attached to the airframe via all mounts. All fuel lines, throttle, and mixture control cables were intact and remained attached to the engine. The top spark plugs were removed and remained free of mechanical damage. The top spark plugs exhibited coloration consistent with normal operation. The propeller was rotated by hand in the direction of normal rotation. No movement of the intake and exhaust rocker arms was observed. In addition, no magneto impulse couplings were heard clicking, and the vacuum pump drive was not rotating. The engine was removed from the airframe and placed on a bench. The left and right magneto, vacuum pump, oil filter, and rear accessory cover were removed from the engine. The left magneto drive gear and the left idle gear shaft were observed loose from their mount. Both bolts were separated from the crankcase, and their respective bolt bores were elongated. Heavy fretting, gouging, and smearing was observed around the mounting surface for the left idler magneto gear drive shaft. Fragmented remains of the safety locking plate and bolts were located within the oil sump. The idler gear shaft, locking plate, and bolts were retained for further examination. The oil filter was cut open, and the internal element was examined. Metallic fragments were observed throughout the oil filter element. The oil suction screen was removed, and found to be free of any significant debris. The left crankshaft idler gear shaft, fragments of the locking plate, and two bolts were subsequently sent to the National Transportation Safety Board Materials Laboratory, Washington DC, for further examination. Examination of the components by a Materials Engineer revealed impact wear and deformation was observed around a mounting hole in the idler gear shaft flange, which is consistent with battering associated with a loose bolt. The locking plate was fractured in several areas and was heavily distorted. The holes in the locking plate were elongated and enlarged. Metal transfer and impact wear was also observed around the holes. The upper and lower ends of the plate were bent 90 degrees to form around the bolt heads. The fracture surface features of the locking plate were found consistent with an overstress fracture. Both bolt heads exhibited similar impact wear damage, however the impact wear damage was most pronounced on the fractured bolt. The hexagonal corners of the bolt heads were rounded, which was consistent with impact wear. The grip region below the head of the fractured bolt was elongated, and the grip diameter was reduced to about 60 percent of its original diameter. This dimensional deformation appeared to have smooth edges consistent with necking. The bolt was fractured in this reduced-diameter area. Around the circumference of the fracture face, the bolt was worn uniformly, consistent with impact wear from metal-to-metal contact. A portion of the fracture had relatively smooth features on flat planes perpendicular to the bolt axis, and exhibited curved crack arrest marks, consistent with fatigue cracking. Ratchet marks were observed around the circumference, consistent with multiple origins. At its deepest penetration, the fatigue crack had propagated approximately 95 percent through the diameter of the fracture surface. The remainder of the fracture exhibited features consistent with overstress separation. There was a visible crack in the grip region of the intact bolt that propagated nearly 200 degrees around the circumference. The axial location of this crack is in a similar location to the fracture in the fractured bolt. This crack was lab fractured to reveal a fracture surface. A small portion of the fracture had relatively smooth features on flat planes, and showed curved crack arrest marks, consistent with fatigue cracking. Ratchet marks were observed over a small area around the circumference, indicating multiple origins. The fatigue had propagated approximately 10 percent across the fracture diameter. The remainder of the fracture exhibited features consistent with overstress separation. Review of the airplane maintenance records revealed that the engine was overhauled on April 7, 1990, at a tachometer time of 248.8 hours. The most recent 100-hour inspection was conducted on March 23, 2010, at a tachometer time of 1,609.5 hours and an engine total time of 3,504.4 hours and 1,373.77 hours since major overhaul. No logbooks entries were observed concerning the idler gear shaft mounting bolts or lock plate.