On March 18, 2001, about 1800 Pacific standard time, a Schweizer 269C, N6123A, rolled over during a forced landing near Ojai, California. The forced landing was precipitated by a loss of engine power in cruise flight. Sun Air Aviation LLC was operating the helicopter under the provisions of 14 CFR Part 91. The private pilot was not injured and two passengers sustained minor injuries. The helicopter sustained substantial damage. The local personal flight departed Camarillo, California, about 1640. Day visual meteorological conditions prevailed, and no flight plan had been filed. The primary wreckage was at 34 degrees 28.51 minutes north latitude and 119 degrees 23.90 minutes west longitude. The pilot submitted a written statement. He was in cruise at 4,700 feet. He planned to fly along a ridgeline northwest of Lake Casitas. As he approached the mountaintop, the nose suddenly veered to the left. He heard several coughing sounds from the engine. He entered an autorotation, but could not reach the flat mountaintop. The helicopter collided with the terrain and rolled. He secured the fuel shutoff valve, pulled the mixture control out, and turned the battery and ignition switch off. He observed fuel leaking from the helicopter. The wreckage was examined by a Safety Board investigator at Ray's Aviation, Santa Paula, California, on May 18, 2001. The investigator established control continuity for the throttle and mixture controls. Both controls moved freely from stop to stop. The bottom spark plugs were removed from each cylinder. The spark plug electrodes were gray in color, which corresponded to normal operation according to the Champion Aviation Check-A-Plug AV-27 Chart. The Safety Board investigator established continuity for the collective and cyclic. The investigator added 4 quarts of oil and set up a temporary fuel supply. The engine started on the fifth attempt. The engine ran smoothly; oil and fuel pressures indicated within the green operating range on their respective instruments. Once the oil temperature rose into the green operating range, the engine was run up to 2,000 rpm and a magneto check was completed. The rpm dropped about 200 rpm when each magneto was selected, and the engine continued to run smoothly. No fuel or oil leaks were observed after engine shutdown. An examination of the engine logbooks revealed that maintenance personnel complied with TEXTRON Lycoming Mandatory Service Bulletin 388B on February 16, 2001, at a Hobbs time of 409.6 hours. This inspection revealed that the exhaust guides for cylinders No. 3 and No. 4 were not within the allowable limit range. The exhaust pushrods for both cylinders were damaged and replaced. Maintenance personnel reamed the exhaust guides in both cylinders as corrective action. The Hobbs meter read 462.6 at the examination. The accident occurred at an estimated 230 hours since the engine was overhauled by a maintenance facility. On June 15, 2001, a Safety Board investigator and the Textron Lycoming representative removed and disassembled all four cylinders. They determined the pushrods were straight. The exhaust valve for cylinder No. 4 appeared different from the other exhaust valves. The number 4 piston was not mechanically damaged. Cylinder No.4 was sent to the Lycoming factory. The No. 4 cylinder exhaust valve was inspected under the supervision of a Safety Board investigator at the Textron Lycoming facility in Williamsport, Pennsylvania, on September 20, 2001. An oversized "go-no go" tool was used to examine the exhaust guide of the cylinder, with no discrepancies noted. The exhaust valve stem was inspected under a microscope and no metal transfer was observed on the stem.