so i have a hotshot header, and the outlet to the cat is 2.5 inches.

here comes the problem. i have a test pipe and an apexi exhaust that are 3 inches.

and the question is, are there any cats that start at 2.5" from the header and expand to 3" at the muffler? because previously i have tried to connect the 2, and experienced leaks at the joints.

 

You probably had crappy gaskets, I have no problems w/ my 3" test pipe meeting up w/ my stock header