A PA25 BANNER TOW ACFT ENG CONNECTING ROD BROKE CAUSING A HOLE IN THE CRANK CASE; AN OIL LEAK; AND FIRE. AN EMERGENCY WAS DECLARED WITH A RETURN TO THE ARPT.

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AFTER A THOROUGH PREFLT; WX CHK AND FUELING; I GOT INTO THE ACFT FOR MY SCHEDULED BANNER TOW. START-UP AND TAXI WAS NORMAL AND I TAXIED TO THE RWY. I PERFORMED A LENGTHY RUN-UP. THE MAGNETO DROP WAS ABOUT 75 RPM WITH THE L MAGNETOS OFF AND ABOUT 50 RPM WITH THE R OFF. CARB HEAT DROP WAS ABOUT 50 RPM. I THEN RAN THE ENG TO NEARLY FULL PWR AND LEANED THE MIXTURE TO SLIGHTLY RICH OF PEAK. I SET THE PWR AT ABOUT 2550 RPM FOR AROUND 15 SECONDS AND LISTENED FOR ANY ABNORMALITIES. EVERYTHING SOUNDED NORMAL AND OIL PRESSURE WAS AT 69 PSI AND OIL TEMP WAS RISING INTO THE GREEN ARC. I RADIOED TWR AND WAS CLRED FOR TKOF AND CLRED FOR AN E BANNER PICK. I TOOK THE RWY AND APPLIED FULL PWR. THE ACFT ACCELERATED NORMALLY AND CLBED AS USUAL. AT 300 FT I MADE A L TURN TO 360 DEGS OVER THE APCH END OF RWY 18L. AFTER CLRING THE RWY AND OVER THE GRASSY AREA I DEPLOYED THE #1 HOOK FROM THE L AND TURNED WBOUND FOR BASE AT 450 FT. I THEN CONTACTED THE GND CREW ON THE HANDHELD RADIO AND TOLD TO PICK THE BILLBOARD ON POLES 1 E. I TURNED S FOR MY BASE LEG AND BEGAN MY DSCNT. TURNING UPWIND AND ALIGNING FOR THE BANNER PICK; MY AIRSPD WAS ABOUT 80 MPH AND RPM ABOUT 1400. OIL TEMP AND PRESSURE WERE NORMAL. I PERFORMED THE PICK PROC AND ACCOMPLISHED PICKING THE BANNER. GND CREW RELAYED 'GOOD BANNER.' I WAS AT 300 FT; 62 MPH AND FULL PWR. TWR CLRED ME FOR A 'SE DEP.' AT 500 FT; I SLIGHTLY RETARDED PWR TO 2500 RPM. AT THE SE BOUNDARY OF THE ARPT I WAS STILL CLBING BUT SLOWLY AT ABOUT 600 FT MSL. I CHKED THE OIL TEMP AND PRESSURE (69 PSI AND 186 DEGS) AND APPLIED NEARLY FULL PWR TO 2600 RPM. ABOUT 5-10 SECONDS LATER THE ENG 'POPPED;' OIL COVERED THE WINDSHIELD; AIRSPD ERODED QUICKLY AND THE ACFT WINGED OVER TO THE L TO NEARLY A 90 DEG BANK. I IMMEDIATELY PUSHED THE NOSE DOWN AND SIMULTANEOUSLY RELEASED THE BANNER. I THEN NOTICED FLAMES AT THE BOTTOM OF THE FIREWALL AND ALONG THE ENG COWL. AT THIS POINT I HAD GAINED AIRSPD BACK TO 65-70 MPH AND WAS WINGS LEVEL WITH A HDG OF APPROX 300 DEGS BACK TOWARDS THE AIRFIELD. WITH THE ENG DEVELOPING LITTLE TO NO PWR; PRODUCING FLAMES; AND SMOKE IN THE COCKPIT; I DECIDED TO PULL THE MIXTURE TO FUEL CUT-OFF. AT THIS POINT THE FIRE AND SMOKE WAS STILL EXTENSIVE AND THE PROP WAS WIND-MILLING. I RADIOED TWR AND EXPLAINED THAT I NEEDED AN EMER LNDG AND WAS TRYING TO MAKE IT TO THE RWY. THE CTLR AUTH ME TO LAND ON ANY ARPT SURFACE. MY GLIDE PATH ALLOWED ME TO FLY TO THE E AIRFIELD BOUNDARY FENCE BY APPROX 50 FT AND LANDED IN THE GRASS BEFORE THE APCH END OF RWY. I AVOIDED THE RWY END LIGHTS AND ROLLED OUT TURNING L (S) ON A TXWY AND CAME TO A STOP. I TURNED OFF THE MASTER AND MAGNETOS; UNBUCKLED AND EXITED THE ACFT TO THE R (W). THE ENG COMPARTMENT WAS STILL PRODUCING FLAMES. I RAN A SAFE DISTANCE AWAY (200 FT) AND CALLED THE MAINT TECHNICIAN. HE TOLD ME HE SAW WHAT HAPPENED AND WAS ON HIS WAY WITH ARPT OPS PERSONNEL. THEY ARRIVED SHORTLY AFTER AND HE EXTINGUISHED THE FIRE WITH A HANDHELD EXTINGUISHER. UPON VISUAL EXAM; A HOLE IN THE ENG CASE WAS FOUND APPROX 6 INCHES BY 4 INCHES AND A PISTON CONNECTING ROD WAS LAYING ON THE BOTTOM OF THE ENG COWL.