Just got the head changed out and it was running great. After driving about 10 miles I heard what sounded like an exhaust leak. Checked under the hood and found the problem. There is a plug/bolt missing from the front manifold. It is located at the bottom of the manifold right above where it connects to the exhaust pipe. It looks like where an O2 sensor would go. But the rear manifold just has a plug in it. I believe mine should only have 2 sensors and I found one right before the cat and one after.
Anyhow my questions are what is this plug called and what size is it and where can I buy one.
Thanks for the help.

 

It's the silver part.

 

Does it have 2 pre cats? if so then 4 O2 sensors.

 

Can only find 1 before and 1 after. If it had 4, wouldn't there be one in the rear manifold too?

 

OK. After 4 hrs of looking, I think I figured out they are called oxygen sensor bung plugs or exhaust bung plugs. Now, where can I buy one?

 

Also found out that a M18-1.5 oil drain plug should work. Guess I'll try calling an exhaust shop today or head to Autozone for a oil drain plug.