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I got my end caps back in April and I had trouble putting them on. The set screws that FM sent were a "new style" that were rounded on the end where it indexes the muffler. I called to inquire about my installation problem and Keith told me that they tried a new style of set screw but it really was not working. He sent me another pair of set screws that had a more pointed end. These worked just fine. A dab of blue and tighten them down to where they just begin (barely) to distort the muffler. Perhaps this is your problem.