On August 23, 2012, about 1630 Pacific daylight time (PDT), a Piper PA-30 B, N222BP, made a gear up landing at Scappoose Industrial Airpark, Scappoose, Oregon. The owner/pilot was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The private pilot was not injured; the airplane sustained substantial damage to the underbelly structure during the landing on the grass next to runway 33. The cross-country personal flight departed Vancouver, Washington, about 1600. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that prior to landing, he lowered the landing gear, but it would not fully extend. He attempted to manually extend the gear but was unsuccessful. The pilot determined that due to his low fuel state he would perform a gear up landing. He elected to land on the soft infield grass adjacent to runway 33 versus on the runway itself in an attempt to minimize the damage to the airplane. After recovery of the airplane, it was determined that the lower fuselage had sustained substantial damage to the structure as a result of the soft ground ripping the aluminum sheet metal and belly frame structure. The airplane was secured for further examination of the landing gear. The airplane was examined by maintenance personnel and an inspector from the Federal Aviation Administration (FAA). During the examination, it was found that there was a failure of the landing gear transmission assembly, which prevented the extension of the nose landing gear. The landing gear transmission assembly was replaced, but the damaged assembly was not retained for further testing due to a miscommunication with the repair facility.