Thinking there isnt a leak and knowing are not the same. Even if its a small leak. Id like to know how you guys are sure. Header to cat side is bolted on using spring and bolt usually. Without a gasket, youre saying the thin edge of the header outlet sits tight against the flared inlet of the cat with no leak. What im saying is that its possible that at the very least it has small leaks from flex movement if not constant small leak because no gasket but maybe too small to carry a noticeable affect on performance, daily driven.