They're designed to be a slip fit for expansion/contraction reasons just like on big diesels. My ATS system has been trouble-free for 23 years, no excessive leakage and same fit as the day I installed it back in '93. There's a slight amount of soot around it, just like on the heavy truck diesels. Of course I bought my kit brand new from ATS so that may make a difference.the problem with the OEM factory turbo is that the pup pipe castings suck.
from age, they are not a correct fit.