Sorry Dan but...
...that extra bung/plug at the cat end of the front pipe is for attaching a CO meter to set the fuel mixture and is a completely different (much smaller) size than the O2 sensor bung. Hence that plug would not work.
The previous poster was correct, the O2 bung is 18 x 1.5 mm thread. What many people don't know is this size is common to about 95% of all cars in the world - even GM, Ford, Japanese, and yes Fiat.
The 02 bung plug the initial poster was looking for is actually a very common parts these days. Many aftermarket pipes and cats have them. Often a pipe for a certain car say has an 02 port for one year but not in the previous year. Instead of making two different pipes the aftermarket supplier just makes the one with the 02 port and supplies a threaded plug for use where it is not required.
Thus when the 02 port is required the installer removes the plug and discards it. I have seen these plugs with a variety of head shapes but they will all work.
So if you are looking for one, just pop into your local muffler shop. Bring him a coffee and I'm sure he will give you a couple. Also, any parts jobber should stock them. They are available seperately from Walker, Maremont, etc.
I would strongly advise you use one of these screw-in plugs instead of welding the hole shut. Maybe someday you will need the bung for a wideband a/f reader for tuning...
Hope this helps. Cheers, Doug
