On June 30, 2008, about 1240 Pacific daylight time, a Cessna 172M, N61736, was substantially damaged during a forced landing near Oakland, California. The commercial pilot and his passenger sustained minor injuries. The airplane was registered to and operated by Flying Vikings Inc. of Hayward, California under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the local traffic watch flight that originated from the Hayward Executive Airport (HYW), Hayward, California at 1100. According to the pilot, during cruise flight, he noticed the engine was running rough along with a surge in power. The pilot adjusted the mixture to the full rich position, which seemed to smooth the roughness. A few seconds later, the pilot noticed that the engine oil temperature gauge was rising and diverted towards the Oakland International Airport (OAK), Oakland. The pilot stated that while en route to OAK, the engine lost power and he initiated a forced landing into an open lot near his position. During the landing roll, the airplane struck a large mound of dirt and came to rest upright. Examination of the airplane by a Federal Aviation Administration inspector revealed that the right wing and fuselage were structurally damaged. Oil was observed on the undercarriage of the airplane from the engine aft to the empennage. On July 24, 2008, the airframe and engine were examined by the NTSB investigator-in-charge at the facilities of Plane Parts, Pleasant Grove, Sacramento. The Lycoming O-320-E2D engine, serial number RL-40510-27A, remained attached to the airframe via all four engine mounts. All of the accessories and cylinders remained attached to the engine crankcase. All four nuts that secured the vacuum pump to the mount pad were found loose. Oil was observed on the rear accessory housing below the vacuum pump and between the left and right magneto. When removed, the oil dipstick indicated no oil within the oil sump. All open lines were plugged with end caps and about 10 psi of compressed air was applied to the oil cooler inlet line. Bubbling oil was observed originating from the vacuum pump mounting pad. The vacuum pump and both magnetos were removed from the engine. The bottom portion of the vacuum pump seal was saturated with oil. The crankshaft was rotated by hand using the propeller and was noted to be extremely stiff. Thumb compression was obtained on all four cylinders. The oil suction finger screen was removed and exhibited metallic debris inside the screen. The number one and three cylinders were removed and examined. Both cylinders exhibited a normal amount of combustion deposits within the cylinder dome. Both pistons were intact and exhibited scuffing on the piston skirt. All internal connecting rods visually appeared to be intact. The number three and four connecting rods along with the general area of the crankshaft exhibited thermal discoloration. The left magneto produced spark on all leads when the magneto drive shaft was rotated by hand with impulse coupling engagement. The right magneto produced spark on all leads when the magneto drive shaft was rotated using an electric hand drill. The propeller remained attached to the crankshaft propeller flange. One of the two blades was slightly bent aft about 10 degrees and the other propeller blade was undamaged. Review of the engine logbook revealed that the engine underwent its most recent 100-hour/annual inspection on June 16, 2008 at a tachometer time of 3,036.9 hours and 3,910.6 hours since major overhaul. A squawk sheet located within the airplane indicated that the vacuum pump was replaced after the most recent flight recorded prior to the accident flight on July 27, 2008. According to the FAA inspector who responded to the accident site, during a telephone interview, the airframe and power plant rated mechanic who installed the vacuum pump reported that he installed the vacuum pump and tightened all the nuts except for the bottom left, which he could not reach. The mechanic stated that he used a screwdriver and hammer to tighten the nut.