On March 8, 2019, about 1000 eastern standard time, a Piper PA-31-325, N45MJ, was substantially damaged when it was involved in an accident near Leesburg, Florida. The pilot was not injured. The airplane was operated by the pilot as a flight test under the provisions of Title 14 Code of Federal Regulations Part 91. According to the pilot, he delivered his airplane to a maintenance facility for an annual inspection. It was suggested by maintenance personnel that an inner landing gear door kit be installed (STC SA00555DE) during the annual inspection. After the door kit installation was completed, numerous test flights were conducted. During the landing gear extension, two green landing lights appeared as expected, but the third light (right main landing gear) was very slow to illuminate. Adjustments were made after each flight, and on the last flight while taxiing back to the ramp after landing the right landing gear collapsed. Examination of the airplane by a Federal Aviation Administration inspector revealed buckling of the right inboard wing. Post-accident extension and retraction test were performed using a hydraulic test unit. Both normal and emergency (hand pump) landing gear extension and retraction test were performed. During normal extension it was noted the landing gear locking time was approximately 25 seconds, which exceeded the manufacturer’s specified time limit; however, the hydraulic test fixture used did not have flow control, which could account for this anomaly. It was discovered when checking the actuator rigging that both left and right main landing gear actuators did not drive the main gears into a locked position. Slight hand pressure on the gear door (inboard force) would keep the main landing gear from locking as actuator piston travel was bottomed out. When the force was removed, the weight of the gear would lock the gear.