Working on a leaner sidecar build requiring rerouting the exhaust,  a smaller pipe would be a real help.    I elected to cut off the faux header shell.   The lower end will receive pie cuts, snaking up toward the swing arm pivot before returning  to the conventional exhaust line; a portion of the outer shell remains at the exhaust port end, with cut tabs to be bent in transition to the remaining inner pipe, TIGed.



Removing the central section of shell exposed a few curiosities.    The last couple inches of the lower inner header pipe is actually stretched a little wider to match the coupling layer sandwiched below the outer shell- and a few inches farther down the pipe is a 5 mm diameter hole (pan over on the second image to view in-post).   Mild surface corrosion around the hole and on the inner surface of the cut away outer pipe testify to the flow of exhaust or vacuum induced moist air.  



My first thought was to look for a corresponding hole near the top of the header pipe, but the portion of outer pipe remaining there prevents inspection; if there is a second hole, no soot betrays the presence- if exaust gasses circulate in the outer exhaust cavity, it looks to be from bottom to top!  Any ideas what the function of that hole is or if there is a matching hole at the leading end?

This is really cool Mavigogun.  Thanks for the pics.  Love the way you sectioned the pipe.  

Gary is absolutely correct.  You need the hole to equalize the pressure.  Without the hole, it would be very difficult to complete all the welds.  The trapped air will heat up as you weld and tend to blow the puddle out.  It also might present a hazard during operation.  Without the hole, the trapped air would heat up and pressurize the cavity.  Given the extreme temperature, it could result in a rupture of the outer pipe or a collapse of the inner pipe.

Thanks again for the photos.