Another possible caution - There is a hole on the bottom of the outer jacket just before it meets the inner header pipe. My assumption is this allows expanded hot air to escape. Blocking that hole will possibly have negative results. 1. Heat build-up.(Blueing) 2. Pressure build-up (Bulging of the outer pipe). 3. Colapse of the inner pipe. -or - 4. Possible colapse of the outter pipe due to vacuum when cooling. 2,3,4 are probably unlikely because it would be pretty hard to get that tight a seal - however, it's remotely possible. 1 is more likely of all to occur. If the fins fit I'd be tempted to drill a corresponding hole in the things to be sure the addition has no adverse consequences. You may think I'm pulling this out of my ******, but I see this fairly frequently in dbl.walled industrial piping (often where heat/pressure extreems are NOT as great as exhaust gas temperatures).
