On August 13, 1994, about 1845 Pacific daylight time, N5063P, a Piper PA-24-180, operated by the owner/pilot, sustained substantial damage after the landing gear collapsed during landing rollout in Nehalem, Oregon. The private pilot and his three passengers were not injured. Visual meteorological conditions prevailed and a flight plan was not filed. The personal flight departed Vancouver, Washington, and was conducted under 14 CFR 91. According to an FAA Aviation Safety Inspector from Hillsboro, Oregon, the pilot stated that he confirmed the landing gear was down and locked during the final approach to runway 15 at the Nehalem Airport. This was corroborated by passenger statements. After a normal touchdown, the airplane rolled out about 100 feet and the landing gear suddenly collapsed, causing substantial damage to the belly of the fuselage. According to the FAA inspector, the main landing gear transmission mounting rivets appeared to have partially separated prior to the landing. After an examination of the separation, the inspector reported the following: The landing gear transmission was pulled out of it's attachment bulkhead between the front seats and under the floorboards. Further inspection reveals that , of the five (5) rivets attaching the transmission mount bracket on each side, (4) had been sheared on the right side and (1) on the left side at some time previously. This was evident by the aluminum oxides present on the ends of the sheared rivets, as opposed to the freshly sheared look of the remaining rivets. It is my opinion that the previously sheared rivets had been cut for some time since there was considerable black metal oxide surrounding each rivet. The aluminum sheet metal bulkhead, separating the transmission from the bracket, appeared to have been recently ripped out. This is to be expected since it was never designed to support the total loads imposed by the landing gear system. The FAA inspector also reported: The maintenance records show that immediately previous to the flight.... the mechanic also adjusted the landing gear down locks.... [The mechanic] stated that he had found the left main landing gear downlock to be excessively tight. He rerigged the downlock to factory specs and retracted the gear three times so that he could recheck the downlock rigging. He found it unchanged and the dimensions OK. The aircraft flew and additional (2) flights with (3) landings prior to it's flight to Nehalem Bay Airport. [The mechanic] feels that the excessive loads imposed on the transmission mounts by the left gear cable could have led to the shearing of the mount rivets. He noted that he has never seen this before, but, recently, the International Comanche Society has advised owners to increase the tension on the downlock for added safety. [The mechanic] disagrees with this advice since it is contrary to the manufacturer's instructions.