MD90 HAD MD80 NOSE TIRES INSTALLED.

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ON TAXI OUT FROM GATE AT SJC; GND ADVISED US THAT OUR COMPANY WANTED US TO RETURN TO THE GATE. I HAD MY FO CONTACT SJC OPS AND INQUIRE WHY. THEY SAID THAT OUR ACFT NEEDED A NOSE TIRE CHANGE. AFTER WE ARRIVED BACK AT THE GATE; I WENT UP TO THE TICKET COUNTER TO CALL DISPATCH AND ADVISE THEM OF OUR RETURN. WHEN I GOT BACK TO THE AIRPLANE; I FOUND 2 FAA INSPECTORS ON BOARD. ONE OF THEM ASKED FOR THE LOGBOOK; AND AFTER INSPECTING IT; ASKED WHY I HAD NOT ENTERED A DISCREPANCY. I TOLD THEM THAT AS FAR AS I WAS CONCERNED; THERE WAS NOTHING WRONG WITH THE AIRPLANE. WE WENT DOWN ON THE RAMP AND INSPECTED THE NOSEWHEELS; AND FOUND NO OBVIOUS PROBS. THE 2 INSPECTORS LEFT; AND WHEN A COUPLE OF OUR MECHS SHOWED UP; THEY TOLD ME THAT THE NOSE TIRES ON THE AIRPLANE WERE MD80 TIRES -- NOT MD90 TIRES. THEY ADVISED ME THAT THE TIRE CHANGE WOULD TAKE ABOUT 30 MINS; SO AFTER CONSULTATION WITH THE GATE AGENTS; WE ELECTED TO KEEP THE PAX ON BOARD. AFTER THE MECHS CHANGED THE FIRST TIRE; THEY FOUND OUT THAT IT WAS FLAT; SO THEY HAD TO CHANGE IT AGAIN. AFTER THEY CHANGED THE SECOND TIRE THEY DISCOVERED THAT THEY DID NOT HAVE THE CORRECT SIZE COTTER PINS TO SECURE THE TIRE THROUGH BOLTS. WE FINALLY DEPARTED THE GATE 2 HRS 30 MINS LATE FOR SNA WITH SOME VERY UNHAPPY PAX.