Join Date: Jul 2016
Location: Seattle, WA, USA
The stock header is 1 1/2" at the head, 2" at the 1st "Y" and 2 1/2" at the 2nd "Y". Then after the "ball joint", is the adapter section which will very in Diam. and length depending on what comes with the muffler.
My bike came with a 2-Bros carbon with a 2" adapter section. If stepping back down from 2 1/2" to 2" sounds counter productive, you get a cookie. That's why just this year I finally got rid of the thing. I went with an SC Project SC1-R. muffler. It's Titanium with a flat back to fit right up beside my rear wheel. The workmanship is excellent and has some Italian styling that really sets it off. SC mostly sells in Europe so you don't see them as much here and that suits me fine. Everybody and his dog has an Akrapovic. I seek out quality stuff that you don't see as often.
I'm a metal fabricator so I made my own adapter by reworking the entire stock header, getting rid of the ball joint and taking the 2 1/2" all the way to the muffler which is 70mm (about 2 3/4"). Any muffler shop can flare the end for you.
While researching the exhaust systems, I found at least 30 outfits making bike exhausts just here and Europe. Go look at the possibilities. That fake Akrapovic is going to be a piece of garbage.