The pilot reported that during a post-maintenance check flight, when he went to retract the landing gear after takeoff, he noticed that the amber light that indicated the gear was fully retracted, did not illuminate. The pilot verified the main landing gear indicator lights functioned and decided to validate in flight that the landing gear would extend. When he extended the landing gear, the green light that indicated the landing gear was down and locked did not illuminate, and the pilot was not able to visually confirm the extension of the main landing gear. After troubleshooting the system and several unsuccessful attempts to extend the main landing gear, the pilot performed a gear-up landing to the dry asphalt runway, with only the nose wheel extended. The airplane slid 500 ft before coming to a stop. The airplane sustained substantial damage to the right horizonal stabilizer and elevator. A postaccident examination of the airplane revealed that a b-nut on the main landing gear actuator gear down hydraulic line was only hand tightened, with visible stripped thread present. The mechanic who performed the annual inspection on the airplane, which included work on the main landing gear system, reported that he forgot to fully tighten the b-nut during the recent maintenance work, which resulted in a loss of main landing gear system hydraulic pressure in flight, and the pilot’s inability to fully extend the main landing gear. A successful main landing gear swing was performed during the maintenance work, but it was not performed after all the maintenance work was completed. Had the main landing gear swing been conducted after the maintenance work was completed, it is likely the loss of the main landing gear system hydraulic pressure would have been identified before the flight.