Exhaust fit-up questions

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I'm 1/3 of the way through a TIG class I'm taking at the local CC. Today one of the jobs was butt welds on 16ga coupons, both with filler rod and fusion welds. While doing the fusion weld I remember a motorcycle exhaust system I had that is advertised as having 'low resistance welds'. Going back and looking at pictures of it, it almost looks as though the pipes were fusion welded. Is this possible, or in any way a good idea? Would the purpose be to minimize the amount of penetrated weldment on the inside of the pipe?Part of my reason for taking the class would be so I could make some basic motorcycle exhaust systems for myself. When fitting up 16ga mild steel tube, should the two pieces be touching each other or should I include a 1/16" gap like on the butt weld I did in class? I assume no edge bevel is necessary either, as my instructor has stated that butt joints done with material less than 1/8" do no require it. He has been very helpful, but I do not want to bother him with questions that are not related to the class at this point.Any answers or help would be much appreciated, I'd like to practice some of this at home when I can.
Reply:The tighter the fit up on thin materials, the better. For tube work less than .120 wall, you want things butted up tight with no gaps.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:Great, thanks for the answer.  Any edge prep? (other than making sure it butts up tightly with no gaps.)
Reply:Originally Posted by WiscoGreat, thanks for the answer.  Any edge prep? (other than making sure it butts up tightly with no gaps.)
Reply:Im sure your instructor, like most others, would welcome questions from a student who is looking to learn things outside the scope of the class.Have we all gone mad?
Reply:I fuse stainless butt welds all the time with 304 fittings. Your getting pretty close to or100% pen with thin tube. be sure to check fit before starting. I like to touch sand the two joints to get intimate  fits......I have pulsed and just plain fused both have good results, watch for undercut........  Miller Dynasty 350Twenty Six HammersThree Crow BarsBig Rock
Reply:Originally Posted by sn0border88Im sure your instructor, like most others, would welcome questions from a student who is looking to learn things outside the scope of the class.
Reply:This is for prototyping car exhaust but will work on motorcycles also, just a bit less complicated on a bike.http://www.stainlessheaders.com/headerfabrication