DHC-6 DIVERTS TO ANOTHER AIRPORT WITH LONGER RUNWAYS DUE TO NOSE GEAR NOT CENTERING AFTER EXTENSION.

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WE BOARDED OUR PAX AT ZZZ1. I THEN DID A PREFLT WALKAROUND PAYING PARTICULAR ATTN TO THE MAIN TIRES AND ASSEMBLIES AS MAINT HAD JUST CHANGED THE L MAIN TIRE. I TAXIED TO RWY 21 FOR DEP MAKING A 90 DEG TURN TO THE R AND A SMALL S-TURN TO FOLLOW THE TAXI LINE TO THE RWY. AFTER BEING CLRED FOR TKOF; I TAXIED ONTO THE RWY MAKING A 90 DEG TURN TO THE L. I TAXIED A FEW FT DOWN THE RWY MAKING SURE THE NOSEWHEEL WAS CTRED. I VERIFIED ON BOTH THE AFTER TKOF CHKLIST AND THE FINAL LNDG CHKLIST THE TILLER WAS CTRED IN ACCORDANCE WITH THE COMPANY PROC. AT NO TIME DID I FEEL OR DID THE ACFT INDICATE TO ME THAT THERE WAS A NOSEWHEEL ISSUE. DURING THE FLARE FOR LNDG ON RWY XL AT ZZZ; WE WERE ALERTED BY A RADIO XMISSION FROM AN AIRPLANE HOLDING SHORT OUR NOSEWHEEL WAS TURNED 90 DEGS. AN IMMEDIATE GAR WAS EXECUTED. WE MADE ANOTHER PASS FLYING OVER THE RWY TO HAVE GND PERSONNEL CONFIRM OUR NOSEWHEEL WAS INDEED TURNED 90 DEGS. ON THE DOWNWIND AND FLY-BY I MOVED THE TILLER TO SEE IF IT WAS MOVING THE NOSEWHEEL. AFTER COMPLETE CONFIRMATION THE NOSEWHEEL WAS STILL TURNED 90 DEGS; WE DECIDED ZZZ WOULD BE THE LONGEST; WIDEST AND SAFEST PLACE TO LAND. THE FO CALLED ZZZ APCH DECLARING OUR EMER. WE DISCUSSED THE BEST WAY TO LAND AND CONFERRED WITH DISPATCH. WE LANDED UNEVENTFULLY ON RWY XR. THE PAX WERE DEPLANED AND THE AIRPLANE WAS TOWED OFF THE RWY. AT NO TIME DURING THIS INCIDENT WERE THE CREW OR PAX INJURED OR ENDANGERED IN ANY WAY. THE FO DID AN EXCELLENT JOB SUPPORTING THE CAPT (PF). ALL COMPANY PROCS AND CHKLISTS WERE FOLLOWED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER ADVISED THAT THE LANDING WAS MADE AT MINIMUM AIRSPEED -- APPROXIMATELY 37 KNOTS -- AND MAX BRAKING AND REVERSE WERE EMPLOYED AFTER TOUCHDOWN. THE LANDING ROLL WAS THUS LIMITED TO ABOUT 25 FEET. THE TIRES REMAINED INTACT AND THE WHEEL CASTERED TO THE CORRECT POSITION QUICKLY. AFTER LANDING; MAINTENANCE DETERMINED THAT GROUND CREW HAD TOWED THE AIRPLANE DURING THE LAYOVER PRIOR TO THE EVENT AND HAD FAILED TO PROPERLY REINSTALL THE NOSE GEAR STEERING PIN. THE GEOMETRY OF THE SYSTEM ALLOWED APPARENTLY NORMAL STEERING AS LONG AS THE WEIGHT REMAINED ON THE GEAR. ONCE AIRBORNE THE OLEO EXTENDED RENDERING THE GEOMETRY INCORRECT TO RE-CENTER ITSELF.