RETURN LAND MANDATED BY AN INFLT ENG SHUTDOWN IN A C-421A GOLDEN EAGLE.

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THE PROB MANIFESTED ITSELF AFTER TKOF CLBING THROUGH 9000 FT AS A CHANGE IN THE SOUND FROM THE L ENG. A QUICK GLANCE AT THE TACHOMETER REVEALED THAT THE L INDICATOR WAS SURGING PLUS OR MINUS 100 RPM. A FEW SECONDS LATER THE L ENG RPM'S STARTED DECREASING SLOWLY AND I STARTED TO LEVEL OFF. I ADVANCED THE L PROP LEVER SLIGHTLY WITH NO APPARENT EFFECT; RPM'S WERE NOW DROPPING VERY FAST OUT OF THE GREEN ARC. THE L PROP WAS QUICKLY FEATHERED AND THE L ENG SECURED. A CALL WAS MADE TO ATC TO INFORM THEM OF OUR SIT AND REQUEST AN IMMEDIATE RETURN TO THE FIELD. ALTHOUGH I DID NOT DECLARE AN EMER; ATC MUST HAVE TREATED IT AS SUCH BECAUSE I HEARD THEM CLRING THE AIRSPACE DUE TO AN 'EMER IN PROGRESS.' THE LONGEST RWY WAS SELECTED AND THE APCH AND LNDG WERE UNEVENTFUL. AFTER INSPECTING THE L ENG; IT WAS DISCOVERED THAT THE PROP SHAFT BEARING HAD FAILED AND THE PROP WAS NOW LOOSE ENOUGH TO MOVE UP AND DOWN ABOUT 2 INCHES. THIS SIT COULD HAVE BEEN EASILY WORSE; HAD IT NOT BEEN FOR THE GOOD TRAINING RECEIVED AND THE QUICK REACTION TO THE MECHANICAL FAILURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR WAS RELIEVED IN HAVING THE CALLBACK MADE. HE HAD BEEN WORRIED ABOUT SOME 'MISINFO' HE HAD SUPPLIED IN HIS ORIGINAL RPT. THE BEARING ON THE PROP SHAFT HAD NOT FAILED; THE PROP SHAFT ITSELF FAILED; CAUSING THE VIBRATION AND RESULTING ENG SHUTDOWN. HE STATED THAT IT DID MAKE A DIFFERENCE IN THE 'CULTURE' OF THE RPT AS A BEARING FAILURE MIGHT HAVE IMPLIED POOR MAINT PRACTICES. SINCE THE PROP SHAFT ITSELF HAD FAILED IT WAS MORE IN LINE WITH A METAL FATIGUE PROB OR MAY HAVE REFLECTED ON THE USE THE ACFT HAD RECEIVED FROM PRIOR OWNERS. THE ACFT HAD BEEN NEWLY PURCHASED AS A USED ACFT ONLY 3 MONTHS PRIOR TO THIS EVENT; ACCUMULATING ABOUT 150 HRS ON THE LOG. HE SAID; IN RESPONSE TO A QUESTION RELATIVE TO MAINT SVC RECORDS; THAT ALL WERE IN ORDER. AS AN ASIDE; THE RPTR WAS ASKED WHY THE CREW HAD NOT DECLARED AN EMER AFTER THE LOSS OF THE ENG. HE SAID THAT THEY FELT IT WAS 'NO BIG DEAL' AT THE TIME AND THAT THEY 'HAD EVERYTHING UNDER CTL.' THIS ANALYST COUNSELED RPTR ON THE BENEFITS RECEIVED FROM DECLARING AN EMER; ONE BEING A PRIORITY IN TFC HANDLING. RPTR AGREED; IN RETROSPECT; THAT THEY HAD ERRED ON THIS POINT.