In this picture I've let the center tube slide down so the air fitting is in position to be welded into place. Once I double check that everything is properly centered, I can weld it. This can be tricky because the air fitting needs to be welded solid to the exhaust pipe, but not to the end cap or center tube. If the air fitting gets accidentally welded to the end cap, it can still be disassembled by unscrewing the 1/2" ID pipe from the end cap. Much easier to slide it off the snug hole in the end cap rather than having to unscrew it all though. Either way will work. If the end cap winds up stuck or welded, the center tube won't come out. It will still work, but stops easy modifications to this for experimentation.

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