Once you have a bung take it to a welder that can do stainless steel and he will take care of it for you. When i did it i think i drilled an 18mm hole and welded the bung on. Then you just screw the o2 sensor into the threaded bung. I had to make a spacer on my lathe so the o2 sensor didnt sit so low in the exhaust pipe and restrict the flow too much.
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