SEA82AA155

While on a sightseeing flt in the Mt. St.helens national volcanic monument area the acft crashed on the mud flats of the north fork of the toutle river in a relatively flat & open area. There were indications of a massive oil leak from the eng area in flight. Eng examination revealed a demounted no.3 cylinder. Hold down bolts were missing from one 1/2 inch stud & one 1/2 inch throughbolt, & from two 3/8 inch studs. There was no evidence of tensile overloading or stripping. One hold down nut of each size was located unattached & captured in the exterior eng baffling. The no. 3 conrod cap was missing. The eng had accrued 446 hrs since overhaul & 24 hrs since last inspection. The wreckage was found near a level straight road free of shoulder obstructions. A surviving pax stated a forced landing on a road would be made. A steep turn was made over the road & the acft "fell." The owner's manual states the plt should be at 1000FT AGL at the downwindposition for a normal approach to a power-off landing. The stall warning system was reported inoperative.