On February 18, 1996, at 1310 central standard time, a Cessna 172, N758CQ, sustained substantial damage during a forced landing due to loss of engine power near Worthington, Minnesota. The private pilot and two passengers reported no injuries. The 14 CFR Part 91 flight departed Worthington Municipal Airport, Worthington, Minnesota, at 1255 cst, en route to Buffalo Municipal Airport, Buffalo, Minnesota. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he departed Worthington, Minnesota, and climbed to 3,500 feet. The first indication that something was wrong was the engine began running rough and there was light smoke in the cockpit. The pilot reported that all the instruments were in the green, the power was set at 2500 RPM with 23 inches of manifold pressure, and the mixture was at full rich. He noticed the oil pressure decreasing and he turned back to land at Worthington Airport. He reported that the engine RPM's began fluctuating, the oil pressure dropped rapidly, and the cockpit began filling with smoke. The pilot decided to secure the engine and execute a forced landing to a gravel road. The airplane touched down on the road, but during landing rollout the right wing hit a yield sign on the side of the road. The airplane swerved into an adjacent field and came to a complete stop. The oil was checked and there was no indication of oil on the dipstick. A two to three inch hole was discovered in the crankcase of the engine. An examination of the engine revealed that the number three piston exhibited a hole burned in the bottom side through the crown structure to the piston interior. The number one and number two connecting rods were off the crankshaft. The connecting rod bolts were broken. The bolts were black and necked down at the fractured area. The number one and two crankshaft journals were partially melted. The number three, four, and five connecting rods were burned and discolored. The crankshaft main bearings exhibited normal operational signatures. The fuel system was checked and no defects were discovered. An examination of the magnetos revealed that the left magneto was contaminated with a blackish debris. The left magneto distributor block interior was coated with the debris. The rear bearing race was found spinning in the left magneto's aluminum housing. Metal particles from the race and housing covered the surface around the magneto distributor block.