A TECHNICIAN RPTS FAA INSPECTOR ALLEGES WRONG PISTONS WERE INSTALLED DURING ENG OVERHAUL. PISTON MODIFIED AND APPROVED FOR ENG UNDER STC.

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A RECENT INCIDENT OCCURRED INVOLVING AN ENG THAT I OVERHAULED IN 2000. THE ACFT INVOLVED IS A FAIRCHILD PT23. THE OWNER TRIED TO START THE ENG AFTER THE ACFT HAD BEEN IN STORAGE FOR 1 YR AND HYD-LOCKED THE ENG RESULTING IN A BENT LINK ROD IN #4 CYLINDER. THE FAA WAS PRESENT AT THE TEAR-DOWN WHERE THIS DAMAGE WAS REVEALED. ALSO PRESENT WAS THE OWNER AND MECH. ALTHOUGH WE HAD REQUESTED TO BE THERE FOR THE TEAR-DOWN; WE WERE NOT ADVISED NOR INVITED TO ATTEND. THE FAA REPRESENTATIVE AND MECH NOTICED THAT THE PISTON I INSTALLED DID NOT MATCH THE PISTON SHOWN IN THE ORIGINAL ENG PARTS MANUAL FOR CONT W670-6A AS THE OIL CTL RING ON THE ORIGINAL PISTON IS IN THE SECOND GROOVE POS; BUT THE PISTON INSTALLED HAS THE OIL CTL RING IN THE 4TH GROOVE. THE FAA IS SUSPECT THAT I USED THE WRONG PISTONS. HOWEVER; THE PISTONS THAT WE INSTALLED ARE A MODIFIED PART; APPROVED UNDER STC SE4127SW ISSUED TO XYZ INC; AND ARE APPROVED FOR USE IN THIS ENG. STC SE4127SW COVERS THE INSTALLATION OF MODIFIED W670-23 PISTON; PART NUMBER 4180; TO REPLACE W670-6A PISTON; PART NUMBER 4177. WHEN I DOCUMENTED THE ENG OVERHAUL IN THE ENG LOG I DID NOT INCLUDE INFO REGARDING THE STC PISTONS OR FURNISH A FORM 337 (MAJOR REPAIR OR ALTERNATION FORM) AS THE PARTS WERE SUPPLIED TO ME AS A FINISHED PART AND INCLUDED APPROVAL TAGS (YELLOW TAGS) FROM XYZ INC; SHOWING THE STC NUMBER AND RETURN TO SVC INFO. THESE PARTS APPROVAL TAGS AND ALL OTHER PARTS IDENT INFO; WAS SAVED IN THE ACFT RECORDS PORTFOLIO. THE OWNERS SAY THEY CANNOT FIND THIS INFO.