I asked about this a few posts ago and got a lot of good response but I have encountered an unforseen problem. I have a 94 pass side manifold I have used and it has a fine thread hole in the aft portion that looks to be a place where the O2 sens would thread in. I had down tubes made without cats and had the O2 sens bung placed on the pass side down tube as per original. I now need to plug this hole in the manifold and was told it was 18mm x 1.5. I ordered a plug from summit and a bold from fastenal. I just got the bolt and it is nowhere near the correct size. Anyone encountered this? I'm now concerned the plug I ordered is wrong too. I'll try to get some calipers on it and measure it shortly.