Here is my Skybolt with a 1.20 AX. I had a pitts muffler when this picture was taken, but I now have the stock muffler. Just use the adapter offered by OS to turn the stock muffler 90 degrees like a pitts muffler. The stock muffler without the noise baffle runs better that the pitts muffler. This is a great combination. I have hundreds of flights on this plane and it definitely has unlimited vertical.