On April 30, 2015, about 1115 central daylight time, a Cessna 152 airplane, N49158, sustained minor damage during a forced landing near Pilot Point, Texas. The flight instructor and student pilot were not injured. Visual meteorological conditions prevailed at the time of the incident. The instructional flight was operated by U.S. Aviation Group, LLC., under the provisions of 14 Code of Federal Regulations Part 91 without a flight plan. The local flight departed Denton Enterprise Airport (KDTO), Denton, Texas, about 1100. The student had just completed steep turns when the engine started running rough; however, the roughness soon stopped. Following another steep turn, the engine started running rough again. The flight instructor went through several procedures and checklists to troubleshoot or diagnose the engine roughness including a magneto check. The rpms fluctuated between 1,200 and 2,000 and the flight instructor was unable to maintain altitude or correct the rough-running engine. He elected to land in a nearby field which resulted in minor damage to the nose landing gear and engine mount. An examination of the engine revealed that the number 2 and number 4 lower sparkplugs were lead-fouled and inoperative and the number 1 and number 3 lower sparkplugs were lead-fouled but still operational. Further examination revealed that the distributor gear finger (part number 510406) on the left magneto (Slick Magneto, serial number 14040289/model number 4381) separated. The distributor gear finger (510406) on the right magneto (Slick Magneto, serial number 14040835/model number 4381) was loose. The magnetos had 478.8 hours total time. Following the incident, U.S. Aviation Group, LLC., examined the remainder of their fleet and discovered an additional 19 magnetos with loose distributor gear fingers. The model numbers were 4381 (same as the incident magnetos), 4370, 4371, 4372, and 4374. Total time on these magnetos varied from 48 hours to 2,168.5 hours. These magnetos were installed new at engine overhaul or as a replacement during maintenance. The Director of Maintenance at U.S. Aviation Group LLC. commented that the method of attaching the finger to the gear was different than in the past. According to the manufacturer, Slick by Champion Aerospace, they had seen 16 magnetos that exhibited similar failure modes: 9 with the separation of the distributor finger and 7 with loose fingers. They added that there was no design change. Controls used in the insertion of the finger into the gear and an additional inspection of this procedure had since been added. On July 2, 2015, Slick by Champion Aerospace issued Service Bulletin 1-15. This service bulletin called for the distributor gear assembly to be removed and replaced on certain Slick Magnetos, at the "earliest convenient maintenance time, not to exceed the next 50 hours." The magnetos involved in this incident were included in the models affected outlined in the service bulletin.