I have my GP extra 300s done, and i'm waiting a pitts muffler for the os 75 AX, my doubt is: should i break in with the original muffler or with the pitts.
thanks

Either way should not matter much. But if you are concerned, break it in with the original muffler. Either way, it won't take much. I ran my OS 75 AX through two full tanks and it was ready to fly. I just left the mixture a tad rich for the first several flights.

You made a great choice of engines. It will surprise you with the power that little gem has over other engines of the same size.

CGr.

thanks, i'm waiting for the pitts in about a month, it still dificult to buy material in portugal, if i buy in spain or UK by myself i already have it

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.