On July 2, 1993, about 0807 eastern daylight dime, N4106M, a Piper PA-12, registered to and operated by Aerial Billboard Corporation, experienced an in-flight fire over Tampa, Florida, while on a 14 CFR Part 91 banner towing flight. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight. The pilot was not injured and the airplane was destroyed by the fire. The flight originated from St. Petersburg, Florida, about 30 minutes earlier. The pilot stated the engine began to run rough, lose power and he observed flames from the cowling. He released the banner he was towing and executed a forced landing to an open field. He exited the airplane without injury and the airplane was destroyed by fire. The engine exhaust system and muffler were removed and forwarded to the NTSB laboratory for examination. Cracking of the inlet pipe sleeve on the heat exchanger was found with evidence that the crack had previously been repaired. The cracking exhibited signatures consistent with long term, progressive, intergranular degradation. The cracking progressed to about 1/3 of the seam and shifted from intergranular cracking to fatigue cracking at the end. This crack allowed the exhaust heat and flames to escape prematurely and impinge upon the fuel line in the area, initiating combustion and fire.