On August 5, 1998, at 1020 hours Pacific daylight time, an experimental Stewart S-51 airplane, N151MT, experienced a loss of engine power approximately 20 miles northeast of Jackson, California. The aircraft, operated by the pilot under 14 CFR Part 91, sustained substantial damage in the ensuing forced landing on Highway 88. The private pilot was not injured. Visual meteorological conditions existed for the flight that departed the Reno, Nevada, airport at 0945. The flight was scheduled to terminate at the San Luis Obispo, California, airport. No flight plan was filed. The pilot stated that 20 minutes into the flight he noted a loss of oil and fuel pressure. The failure occurred at 5,500 feet above ground level (agl) and the nearest airport was in excess of 20 nmi away. He stated that he chose an unoccupied road to make the emergency landing. After touchdown, the airplane collided with a snow pole and veered off of the road and came to rest after colliding with a tree. The Federal Aviation Administration interviewed the owner/builder of the kit. The engine is a General Motors 502-cubic-inch displacement automotive powerplant, which was modified for installation in the airframe. The engine accessories are driven by a "V" belt system. The owner stated that he modified the drive shaft from the original "V" belt drive to a dual "serpentine" belt/pulley system. He further stated that in order to install the new system, he had to weld the new serpentine drive to the original drive shaft. He installed and welded a retaining pin on the center of the drive shaft. An examination of the airplane was conducted by a FAA inspector. He found the propeller governor pulley at the bottom of the engine compartment, with the drive shaft broken at midpoint where the retaining pin had been welded. The inspector noted a bluing and discoloration of the shaft. He further noted that the original manufacturer had placed a heat treat stamp on the shaft and the integrity of the shaft may have been compromised by the owner/builder's welding of the shaft.