Ok. I would just go ahead and break it in using the stock muffler. That way you will be all ready for when the Pitts comes in. Are you just putting the pitts on for looks or is it because the stock muffler doesn't clear? If it clears, and you don't mind it that way until your Pitts comes in, go ahead and fly it.

That is one outstanding engine. I have it on a Goldberg Protege. Minnflyer did a review of that model with the OS 55 AX engine and it flew great with that. Mine balanced perfectly with the 75 AX and flew just great. A real performer.

CGr.