PA28 PLT HAD A SINGLE DRIVE SHAFT GEAR FAILURE ON A DUAL MAGNETO THAT CAUSED ENG FAILURE.

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I AM WRITING THIS TO EXPLAIN A FAILURE THAT I EXPERIENCED ON MY PIPER DAKOTA. I WAS FLYING FROM HWV TO N87. WHILE ENRTE (NOT TOO FAR FROM MY DEST); I HEARD A NOISE THAT SOUNDED LIKE A RATTLING (MY THOUGHT WAS THAT IT MIGHT BE AN INSPECTION PLATE THAT HAD COME LOOSE). I STARTED A CLB (WITHOUT CHANGING PWR SETTINGS) JUST TO GIVE MYSELF MORE OPTIONS. ONCE OVER THE ARPT; I CALLED IN AND SPOKE TO A PLT THAT WAS AT THE APCH END OF THE RWY; AND ASKED IF HE COULD LOOK AT MY PLANE TO INSURE EVERYTHING LOOKED OK; AND EXPLAINED THAT I HEARD A NOISE AND THAT I AM NOT SURE WHAT IT MIGHT BE. I REDUCED PWR OVER THE ARPT (SINCE I WAS EXTREMELY HIGH AT THIS POINT) AND DID A WIDE TURN TO SET UP ON DOWNWIND. ONCE IN FLAP RANGE; I ADDED 10% FLAPS AND PROCEEDED TO TURN BASE; AND THEN FINAL. AT THIS POINT; I FELT THAT IT WAS THE ENG; SINCE IT DID NOT FEEL RIGHT; EVEN THOUGH ALL INSTS WERE STILL READING OK. I PULLED PWR TO IDLE AND THE PLT ON THE GND INDICATED THAT NOTHING LOOKED WRONG; AND I LANDED WITH NO INCIDENT. ON THE RWY; AS I WAS SLOWING DOWN; I NOTICED MY MANIFOLD PRESSURE WAS VERY HIGH. AFTER I PULLED OFF THE RWY; I THEN ATTEMPTED TO ADD PWR (JUST TO SEE HOW IT WOULD RESPOND) WHICH HAD NO EFFECT OTHER THAN A SLIGHT CHANGE IN THE ENG SPD AND A FURTHER INCREASE IN MANIFOLD PRESSURE. I PROCEEDED TO MY DEST ON THE ARPT AND SHUT THE ENG DOWN. WE DID A VISUAL INSPECTION ON THE ENG AND NOTICED 2 CYLINDERS WHERE HOT; WHILE MOST OF THE OTHERS WERE EITHER SLIGHTLY WARM OR COLD. A MECH LATER REMOVED MY MAGNETO; AND DISCOVERED THAT SOME TEETH ON BOTH GEARS (L AND R) WERE MISSING. THE ENG WAS NOT CAPABLE OF RUNNING AT THIS POINT. THE MAGNETO WAS REPLACED BACK IN APRIL WITH A REBUILD UNIT. THE COMPLETE FAILURE OF THE MAGNETO OCCURRED AFTER I HAD LANDED DURING TAXI; SINCE I WAS FLYING UNDER PWR RIGHT UP TO THE TIME I DECREASED PWR. THE FAILURE TOOK TIME BEFORE IT MADE THE ENG INCAPABLE OF PRODUCING PWR; AND I LANDED UNEVENTFULLY. I ALSO BELIEVE THAT HAD I CHANGED PWR SETTINGS DURING FLT I MIGHT HAVE LOST PWR EARLIER ON. THIS IS A SINGLE SHAFT DUAL MAGNETO.