My muffler actually came with two orings, one for each side of the center section. My center section is made of aluminum. I cut it down to fit my plane. Havent ran the engine yet to tell if it is going to leak. it was posted earlier in this thread that removing the center section would reduce performance. The guy I work for said as soon as he looked at it that it would not stay tight. Seems the different types of metal will expand differently causing the main through bolt to come loose.
Jeff