A FLT INSTRUCTOR RPTS FLYING A C172 OUT OF 100 HR INSPECTION TIME AFTER BEING TOLD THE INSPECTION HAD BEEN COMPLETED IN CONJUNCTION WITH AN ENG CHANGE.

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AFTER THE FLT SCHOOL; WHERE I HAD TAUGHT FOR 8 YRS; CLOSED; I TOOK 2 OF MY STUDENTS TO THE SCHOOL NEXT DOOR TO FINISH THEIR TRAINING. IT WAS TO BE OUR VERY FIRST FLT THERE. I JOINED MY STUDENT AS SHE WAS FINISHING UP THE PREFLT ON THE CE172N. AT MY PREVIOUS SCHOOL; ACFT ARE DISPATCHED WITH A CLIPBOARD; ON WHICH IS A STICKER DETAILING THE REQUIRED INSPECTIONS (ANNUAL; 100 HR; XPONDER; ELT; ETC). I TEACH MY STUDENTS TO ALWAYS CHK THEM; AND I RECHK THEM WHENEVER I GET IN THE AIRPLANE. THE NEW SCHOOL DISPATCHES A 3-RING BINDER WITH THE ACFT; WHICH INCLUDES A PAGE DETAILING THE INSPECTIONS. I OPENED THE BOOK TO CHK THAT THE ACFT WAS LEGAL AND FOUND THAT ONLY LITTLE MORE THAN 1 HR (TACH TIME) WAS LEFT UNTIL THE 100 HR INSPECTION WAS DUE. NOT WANTING TO OVERFLY THE 100 HR OR THE REQUIRED AIRWORTHINESS DIRECTIVES; I WENT INSIDE AND BROUGHT THIS TO THE ATTN OF THE SCHOOL'S OWNER. AS IT TURNS OUT; THE OWNER HAD RECENTLY SOLD THE ACFT TO AN INDIVIDUAL WHO WAS THEN LEASING IT BACK TO THE SCHOOL. THE NEW ACFT OWNER DECIDED TO HAVE A NEW ENG INSTALLED AND A FRESH 100 HR DONE. I WAS TOLD THAT THE ACFT OWNER FLEW THE ACFT FOR THE 25 HR BREAK-IN PERIOD AND THAT THERE WAS ROUGHLY 75 HRS REMAINING TILL THE NEXT 100 HR INSPECTION. THE SCHOOL OWNER STATED THEY WOULD UPDATE THE FIGURES IN THE DISPATCH BINDER. I FLEW THE ACFT THAT DAY; AND ON SEVERAL SUBSEQUENT LESSONS. TODAY IT WAS DISCOVERED THAT THE 100 HR INSPECTION WAS NOT ACCOMPLISHED WHEN THE ENG WAS INSTALLED! THEREFORE; I VIOLATED THE REGS BY FLYING THIS ACFT. I HAD RELIED ON THE INFO GIVEN BY SOMEONE INTIMATELY FAMILIAR WITH THE ACFT. HAD I INSISTED ON EXAMINING THE LOGBOOKS; I WOULD HAVE CAUGHT THE DISCREPANCY.