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I need to place an O2 sensor in my exhaust manifold. My exhaust manifold is cast iron and I cannot weld a bung on to it. I've also looked around and the shops around me aren't able to weld onto cast iron as well.

I noticed there is a flat spot on the manifold near the flange to the y-pipe, that would be an ideal location for a sensor. I'm wondering if it would be possible to drill and tap an 18mm x 1.5 threaded hole this spot. Is it possible to drill & tap a cast iron manifold easily, and are there any special precautions I would need to take to prevent damaging the manifold?

Are there any other options for getting an O2 sensor that I have that I'm overlooking?

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  • My first question is, why? Does the car have an O2 sensor already, but which is jammed in (in which case, weld the new bung nearby), or are you adding an emissions system to a car that doesn't already have one?
    – PeteCon
    Sep 26, 2016 at 18:23
  • @Pete My new ECU requires an O2 sensors for each manifold. Previously it only had one on the driver-side manifold.
    – Kevin Evans
    Sep 26, 2016 at 18:25

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You can drill and tap a cast iron, yes, only difference - it is more fragile than steel, but it should not slow you down. Other option is to weld it in lower part, where the steel pipe starts. It doesn't really matter where you place it as long as it is before Catalytic Converter. In fact, if you place it in one of manifold pipes it will be a little restriction for that cylinder. But it's minimal. Drill it in manifold if your heart says so :)

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