CESSNA SUFFERS SEVERE PWR LOSS AND ROUGH RUN. DECLARES EMER AND DIVERTS.

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DEPARTING ZZZ1; WE HAD JUST FINISHED OUR XING S OF ZZZ2 ARPT AND HEADED S TO ZZZ3 AND BACK TO ZZZ4. NBOUND; I WAS JUST TURNING THE CORNER AND THE ENG INSTANTLY STARTED RUNNING ROUGH; VERY ROUGH. I THOUGHT WE WERE GOING TO LOSE IT ALTOGETHER AT 1000 FT AGL OVER A DENSELY POPULATED AREA. I IMMEDIATELY TURNED S AGAIN TOWARDS THE LAST ARPT WE HAD PASSED AND CALLED THE TWR; 'ZZZ TWR; C172 IS COMING IN.' 'C172; WHAT IS THE NATURE OF YOUR TROUBLE?' 'ENG RUNNING ROUGH.' 'HOW MANY SOULS?' (I HATE THAT QUESTION BUT THEY NEED TO KNOW HOW MANY AMBULANCES TO ROLL.) '2.' 'HOW MUCH FUEL?' '40 GALLONS.' 'ARE YOU DECLARING AN EMER?' 'YES.' 'WOULD YOU LIKE FIRE RESCUE?' 'YES.' 'I HAVE PUSHED THE BUTTON.' I TRIED THE CARB HEAT; NO RESULTS. MIXTURE FULL IN; AND I USED OUR EXCESS AIRSPD TO CLB. THIS ADDED ABOUT 200 FT TO OUR ALT; VIOLATING THE CLASS B AIRSPACE 1500 FT MSL FLOOR. OIL PRESSURE WAS READING OK AS WAS OIL TEMP. I TRIED DIFFERENT PWR SETTINGS; NO LUCK. HERE WE WERE AT 70 KIAS 'FULL' PWR AND BARELY HOLDING ALT. I CALLED ON THE COMPANY AIR-TO-AIR TO RPT I WAS GOING DOWN. OUR OTHER FIXED WING IMMEDIATELY TURNED TOWARDS OUR POS. I WAS NOT SURE HOW LONG THIS WOULD LAST; SO BOTH MY OBSERVER AND I WERE LOOKING FOR A PLACE TO LAND SHORT OF THE FIELD. I AM USUALLY LOOKING FOR PLACES TO LAND AS WE ARE CRUISING AROUND. NEVER HURTS. INTERSTATE WAS LIGHTLY LOADED AND WAS A REAL POSSIBILITY. THE COUNTY HWY WAS A POSSIBILITY AS WELL. WE ALSO HAD A COUPLE OF LARGE FIELDS ON OUR WAY. THE ENG HELD AND WE WERE CLRED TO LAND; WIND CALM. AS WE ROLLED OUT; WE WERE MET BY THE ENTIRE FIRE CREW. SINCE THE EVENT WAS SUDDEN; I HAD NO IDEA HOW BAD THE DAMAGE WAS AND WHETHER WE WOULD HAVE A FIRE UPON LNDG. IT TURNS OUT WE WERE NOT LEAKING ANY OIL OR FUEL. WE PUSHED THE AIRPLANE OFF THE RWY; THEY HAVE NO TOW BARS FOR SMALL GA AIRPLANES. IT WAS PARKED WITH THE OTHER FIXED WING ACFT ON THE FIELD. THE AIRFIELD MGR CAME OUT AND GREETED US. WHILE I WAS WAITING FOR MY BOSS AND THE MECH TO ARRIVE; I SAT IN THE FBO LOUNGE WATCHING TV AND FILLING OUT THE MOUND OF PAPERWORK FROM AN UNAUTH LNDG. I STAYED WITH THE PLANE IN HOPES WE COULD FIX IT AND FLY IT OUT TODAY. THE PLANE TURNED OUT TO BE BROKEN HARDER THAN I THOUGHT. I ORIGINALLY THOUGHT THE MAGNETOS (DUAL DRIVE) HAD SLIPPED A GEAR SINCE THE ENTIRE ENG WAS RUNNING BADLY. IT BECAME VERY OBVIOUS WHEN WE REMOVED THE COWLING THAT WE HAD A PROB WITH THE #3 CYLINDER. THE INTAKE PUSH ROD WAS BENT. THIS SEEMED STRANGE UNTIL WE REMOVED THE ROCKER COVER. THE STUD FOR THE EXHAUST VALVE HAD COME LOOSE IN THE HEAD AND THE ROCKER WAS COCKED TO ONE SIDE. THE PUSH ROD WAS NOT EVEN CONTACTING THE ROCKER ARM. THIS TRAPPED THE HIGH PRESSURE COMBUSTION RESULTS IN THE CYLINDER. WHEN THE INTAKE VALVE TRIED TO OPEN IT COULDN'T. TOO MUCH PRESSURE. THE PUSH ROD BENT. NOT ONLY DID WE HAVE A DEAD CYLINDER; WE HAD THE REST OF THE ENG PUSHING AGAINST A FULL COMBUSTION CHARGE THAT COULD NOT ESCAPE. THE MECH AND I REMOVED THE OFFENDING CYLINDER AND RETURNED TO HOME BASE.