What was found
UPON A ROUTINE ENGINE COWLING REMOVAL, RT ENG, AN EXHAUST STACK ATTACH NUT, WASHER AND BOLT WERE FND LOOSE IN COWLING. FURTHER INSP SHOWED 50 PERCENT OF ALL INBD AND OTBD EXHAUST STACK ATTACHMENT NUTS FREE RUNNING AND A TOTAL OF 90 PERCENT HAD UNSATISFACTORY SELF-LOCKING FRICTION. EXH STACKS HAD BEEN REMOVED/RE-INSTALLED 7 TIMES SINCE FACTORY NEW AND NUTS WERE ORIGINAL. SUSPECT CAUSE COULD BE THERMAL CYCLING TO AFFECT NUT LOCKING FEATURE. RECOMMEND BACKING OFF NUTS TO CHECK LOCKING FEATURE AND TORQUING TO SPECIFICATION. ALSO, CONSIDER REPLACING NUT ANY TIME ONE IS REMOVED.