Slide the center tube inside the exhaust pipe as shown. It should connect with the air fitting welded near the bend in the pipe and line up centered. For my application I've screwed in this 3/8" elbow for the connection for the untreated air from the bubbler to enter. This is why I threaded the ID of that 1/2" ID pipe for the 3/8" NPT. This way the exhaust gases can pass unhindered. Also you'll notice the exhaust pipe will increase in size at this location after completely assembled.

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