I usually don't do under-car welding (but instead usually refer people to muffler shops where they have vehicle lifts and are better set up for such work), but in this case I made an exception, since it was a VW vanagon with the engine and exhaust right at the rear of the vehicle.

The customer had a Westfalia VW camper van that he needed to get a muffler on, to be legal and also to make it quieter. (I don't know what the story was with the previous muffler, but it looks like it was ripped off, perhaps it was backed into and got caught on a rockery, or something? )
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He brought three mufflers over, and the only one that was narrow enough diameter to fit was a glasspack. (Which is essentially, a straight-through design with resonator chamber that has fiberglass or steel wool tucked into it.) Luckily it was a pretty decent sized length so it will at least provide some muffling.
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I welded a bushing over the existing exhaust which was a little bit shy of the proper OD, where it was pinch-bent and especially small on the outside diameter. That gave something very firm to clamp to and should seal the exhaust gasses. I coulnd't weld around the top without removing it so i just welded around the bottom of both sides of the bushing. Here is what it looked like after welding:
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I gave it some spray with cold zinc galvanization to help the old, rusty mild steel components last a little longer. Here is a picture of a muffler hanger/support piece I fabricated:
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I used 1/16" tungsten and the torch switch to control the heat. Since I was welding old nasty metal, I ditched the gas lens and used a regular collet body. (I find that the mesh screens of the gas lens collet bodies can easily get messed up welding dirty metals.)