On July 2, 2004, about 1936 eastern daylight time, a Cessna 421B, N421KS, was substantially damaged while landing at Portland International Jetport (PWM), Portland, Maine. The certificated private pilot and passenger were not injured. Visual meteorological conditions prevailed, and the airplane was operating on an instrument flight rules flight plan. The personal flight, which departed Groton-New London Airport (GON), Groton, Connecticut, at 1730, was conducted under 14 CFR Part 91. According to a written statement submitted by the pilot, prior to his departure from Groton, the airplane was involved in "thunderstorm activity" while parked on the ramp. Line personnel reported to him that heavy wind and rain "hydroplaned" the airplane across the ramp about 25 feet. Prior to departing Groton Airport, the pilot performed a preflight examination of the airplane and found no abnormalities. After takeoff, the pilot retracted the landing gear and noticed an unfamiliar "clunking" noise during the retraction cycle. The climb, cruise, and decent portions of the flight were all uneventful. During the approach to Portland Jetport, he extended the landing gear and noted all three of the landing gear down and locked indicator lights were green. The airplane touched down on the main landing gear, and as the airplane decelerated, it touched down softly on the nose wheel. Sometime thereafter, the pilot heard the "clunk" noise again, and the right wing began to "settle down lower than normal." The airplane then began to veer to the right side of the runway and came to rest in the grass on the right side of runway 29. Examination of the right main landing gear revealed that the upper right landing gear trunnion ear and the attaching drag brace end ear had broken. The parts were submitted to the Safety Board Materials Laboratory. Metallurgical examination of the mating fractures revealed features typical of overstress separation. The fractures contained no evidence of fatigue. The weather reported at Groton, on the day of the accident at 1649, included winds from 190 degrees at 7 knots gusting to 33 knots, and 3/4 statute miles visibility in heavy rain.