In this picture you can see I've inserted the air fitting into the hole in the end cap. I will slide this downward in the exhaust tube until the air fitting drops down snuggly into the hole in the exhaust tube that I've carefully contoured for easy welding. Notice I've taped the 1/4 NPT of the air fitting to protect it some from damage when I weld it. If I have done a proper job, everything will be centered perfectly in the pipe.

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