On December 23, 2016, about 1618 eastern standard time, an Aeronca model 15AC airplane, N1337H, collided with a snowbank and nosed over while landing at Oakland County International Airport (PTK), Pontiac, Michigan. The private pilot was not injured, and the airplane sustained substantial damage. The airplane was registered to Samcran LLC and operated under the provisions of Title 14 Code of Federal Regulations Part 91 without a flight plan. Day visual meteorological conditions prevailed for the local flight that departed PTK about 1500.The pilot reported that the purpose of the flight was to practice landings in the tailwheel-equipped airplane. After departure, he remained in the airport traffic pattern and completed 14 uneventful touch-and-go landings before deciding to conclude the flight with a full stop landing on runway 27R. He reported that he made an uneventful full stall (three point) landing on the runway centerline. However, shortly after touchdown, the airplane suddenly swerved to the left. He attempted to regain directional control with an application of right rudder and increased engine power for an aborted landing. The airplane then swerved to the right as he simultaneously applied forward stick pressure to get the tail airborne. The airplane continued to swerve right until it collided with a 3 ft high snowbank located alongside the runway. The airplane then nosed over and came to rest in the snow-covered area located off the right side of the runway. The pilot was able to exit the airplane uninjured after releasing his lap belt. The pilot had accumulated 205 hours in the airplane since he purchased it in May 2015. The pilot noted that most of his flight experience in the airplane, 172.9 hours, had been flown with the airplane equipped with landing floats instead of a conventional (tailwheel) landing gear. He had logged 32.1 hours of tailwheel time, all of which were flown in the accident airplane. The pilot received his tailwheel endorsement on March 11, 2016. A postaccident examination by a Federal Aviation Administration (FAA) airworthiness inspector revealed substantial damage to both main wing spars and the rudder. The FAA inspector reported that the four 0.25 inch anchor bolts used to attach the left brake torque plate to its respective landing gear axle endplate had sheared. The unrestrained torque plate allowed the brake assembly to rotate with the brake disk that was attached to the wheel. With the forward rotation, the brake line was pulled around the gear leg until the 90° aluminum fitting that connected the brake line to the caliper assembly sheared. The tire inner sidewall exhibited a gash that partially exposed the inner tube. The inner tube did not rupture but was partially protruding from the gash in the tire sidewall. The sheared anchor bolts had worn a grove on the inboard surface of the left brake disk. According to maintenance documentation, on January 27, 2016, the airplane had been modified through the implementation of STC No. SA1114NW with Cleveland 8.00 x 6.00 main wheels, tires, and 6 inch external, single cylinder, dual piston brake assemblies. The landing floats were reinstalled on an unknown date during 2016. On December 17, 2016, the landing floats were removed, and the main landing gear and wheels were reinstalled to convert the airplane back into a conventional gear land airplane. The accident occurred during the second flight after the airplane was reequipped with the conventional landing gear. The installation instructions for STC No. SA1114NW specified that the stock Cleveland brake torque plates be modified per the accompanying installation drawing No. 1200-4 and to use 0.375 inch outside diameter (OD) to 0.25 inch inside diameter (ID) bushings. Installation drawing No. 1200-4 specified that each torque plate required two 0.25 inch holes to be marked and drilled to ensure proper alignment with their respective landing gear axle endplate. Each torque plate was then attached to its respective landing gear axle endplate with four 0.25 inch anchor bolts and washers. The two 0.25 inch holes drilled in each torque plate did not require bushings when installed with a 0.25 inch anchor bolt; however, the remaining two 0.375 inch holes used to attach each torque plate required 0.375 inch OD to 0.25 inch ID bushings to properly support the 0.25 inch anchor bolts. Further examination of the airplane revealed that both torque plates were not modified and installed in accordance with the STC No. SA1114NW installation instructions. Specifically, the supplied brake torque plates were not modified with the two additional 0.25 inch holes to ensure proper alignment with the landing gear axle endplate. Additionally, both torque plates were installed with improperly sized bushings in multiple holes. The left torque plate, which had separated from the left landing gear axle endplate, had two bushing remaining; one bushing had the correct 0.25 inch ID, the other bushing had a larger 0.3125 inch ID. The right torque plate, which remained attached to the right landing gear axle endplate, was incorrectly installed with four 0.3125 inch ID bushings.