A PA23-250 AT 3000 FT ON 10 MI FINAL HAS L ENG FAIL WITH SMOKE IN THE COCKPIT CAUSED BY A L MAGNETO MOUNT BOLT INSTALLED FINGER TIGHT.

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I WAS ASSIGNED A HDG OF 340 DEGS AT 3000 FT UNTIL ESTABLISHED ON THE ILS RWY 35R AND CLRED FOR THE APCH WHEN I NOTICED THE NEEDLE SWING THROUGH THE HSI. AS I TURNED TO JOIN THE LOC; I HEARD A POP AND THEN A SPOTTER; AND FELT A SLIGHT YAW JUST LIKE IN TRAINING WHEN YOUR INSTRUCTOR SHUTS OFF THE FUEL SELECTOR TO SIMULATE AN ENG FAILURE. AFTER LEVELING OFF; I NOTICED THE DIRECTIONAL GYRO HAD PROCESSED; SO I RESET IT AND TURNED TO CORRECT MY COURSE. APCH THEN INSTRUCTED ME TO CONTACT TWR. TWR CLRED ME TO LAND AND WORRIED ABOUT THE RPTED FREEZING RAIN. I ASKED ABOUT THE RWY CONDITION. THEY TOLD ME IT WAS JUST WET. ABOUT THAT TIME THE GS WAS ABOUT TO CTR; SO I LOWERED THE LNDG GEAR AND STARTED DOWN THE ILS. I CHKED TO MAKE SURE THE FUEL CTLS WERE IN THE ON POS WHEN I NOTICED SMOKE FILLING THE COCKPIT FROM THE FLOOR. I QUICKLY SHUT THE HEAT OFF; BUT CHOSE TO LEAVE THE MASTER SWITCH ON; SINCE I NEEDED THE ELECTRICITY TO FINISH SHOOTING THE APCH. I DECIDED IT WAS BETTER TO FLY THE AIRPLANE AND WORRY ON THE GND. THE SMOKE HAD NOW GONE AND EVERYTHING SEEMED UNDER CTL. TWR THEN CLRED ME TO TAXI TO PARKING; SO I TAXIED IN AND NEVER RPTED A PROB. THE ACFT HAD JUST BEEN INSPECTED FOR AN AIRWORTHINESS DIRECTIVE ON A BOLT ON THE ENG ACCESSORY GEAR. BECAUSE OF A DISTR/MISUNDERSTANDING; THE NUTS ON THE L MAGNETO WEREN'T TORQUED. THE ENG WAS RUNUP AND TESTED OK. IT ALSO CHKED OK DURING MY BEFORE TKOF RUNUP. DURING FLT; THE NUTS GRADUALLY LOOSENED; CREATING AN OIL LEAK AND EVENTUALLY ALLOWED THE MAGNETO TO FALL OFF OF THE ENG. ONCE IT WAS DRY OF OIL; THE ENG BEGAN TO SEIZE AND THREW A CONNECTING ROD(S). SMOKE FROM THE ENG WAS THEN CARRIED IN THE SLIPSTREAM AND INTO THE FRESH AIR VENT IN THE TAIL; THUS EXPLAINING THE SMOKE IN THE COCKPIT.