OK well i have searched and everyone says its the M18x1.5 bolt that will fit in there, but on my header it wont fit. I have a o2 sensor in there right now that was cut but its rubbing against the oil pan. Anyone else have this problem. By the way its a 94 4 door GSR with a 4-2-1 DC sports ceramic header. I need to plug the extra o2 bung hole because mine is located on the cat not the header...any suggestions?

 

Wrong header? I had one for my GSR & it didn't have a bung at all. Maybe you got a LS header, so the A-pipe is hitting the pan, not the sensor...

Anyway, I thought maybe it was M18x1.25 but I'm not real sure about that right now.

 

nope its the right header. It just has a extra bung sensor. I can tell its hitting the pan at low rpms because the passanger side lower tranny mount near the tire is broken and the engine moves a bit at low rpms. The sensor is also really close to the oil pan...i mean like maybe a couple millameteres away. So yeah i know its not a M18x1.5 because he tried it and said it was too big...so i bought another one and compared it to a spark plug which is the same size and its too big. I am thinking its a M16.