AN A AND P MECH WITH INSPECTION AUTHORIZATION FOUND THAT HE HAD MISSED A MANDATORY INSPECTION ON 4 MAGNETO COUPLINGS BECAUSE THE ENG MANUFACTURER HAD CHANGED THE REQUIREMENTS WITHOUT HIGHLIGHTING THE CHANGE.

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I WAS DUE TO START AN EVENT OF A PROGRESSIVE INSPECTION IN A DAY OR SO AND WAS CHKING TO SEE WHAT SPECIAL INSPECTIONS NEEDED TO BE COMPLIED WITH DURING THIS EVENT. IN SO DOING I REMEMBERED HAVING RECEIVED A CUSTOMER INFO BULLETIN FROM THE ENG MANUFACTURER COUPLING WHICH WOULD ELIMINATE THE REPETITIVE INSPECTION REQUIREMENTS OF THEIR SVC BULLETIN X. I THOUGHT I MIGHT TAKE THE OPPORTUNITY TO INSTALL THE NEW STYLE COUPLINGS. I HAD BEEN USED TO COMPLYING WITH AD78-09-07 WHICH REFS SB X AND BASICALLY REQUIRES INSPECTION OF THE IMPULSE COUPLING EVERY 500 HRS. AS A FAR PART 135 AIR TAXI OPERATOR; WE ARE REQUIRED NOT ONLY TO COMPLY WITH AIRWORTHINESS DIRECTIVES BUT WITH MANUFACTURERS' RECOMMENDATIONS (SVC BULLETINS) AS WELL. WHEN THE ENG MANUFACTURER ISSUED THEIR CUSTOMER INFO BULLETIN OUTLINING THEIR NEW IMPROVED IMPULSE COUPLING; THEY ALSO ISSUED AN UPDATE TO THEIR SVC BULLETIN; SB X. WHEN I FIRST RECEIVED THE SB I DIDN'T LOOK AT IT VERY CAREFULLY. SB X-D; MUCH TO MY DISMAY AND SURPRISE; WAS CHANGED TO REQUIRE THE INSPECTION OF THE IMPULSE COUPLING EVERY 100 HRS. I WAS SHOCKED TO FIND THE CHANGE AFTER ALL THESE YRS (THE ORIGINAL AD IS FROM 1978) AND DISCOVERED I HAD ALLOWED THE 4 IMPULSE COUPLINGS ON OUR AFFECTED ACFT TO GO UNINSPECTED FOR ABOUT 100 HRS BEYOND THE REQUIREMENT. I IMMEDIATELY PERFORMED THE REQUIRED INSPECTION PRIOR TO FURTHER FLT. ALL 4 IMPULSE COUPLINGS PASSED THE INSPECTION WITHOUT ANY PROBLEM. FUTURE EVENTS LIKE THIS COULD BE ELIMINATED BY HAVING ISSUERS OF SVC BULLETINS SOMEHOW HIGHLIGHT CHANGES THAT ARE MADE IN BULLETINS THAT ARE REVISED.