I got both the header pipe and cat converter from walker. My old cat con is 36 3/4 and new ones 36 1/2 not much difference. O2 sensor in on the cat converter not having a problem there. There may be some size difference with the pipe, and may be the problem. Trying to find a longer one but I am having a hard time finding any differences, not sure if i should try another brand or not. The flange connections seem fine. I got my manifold from salvage yard, no problem there. I checked and double checked all product fittings and recommended brands.