I bought a HKs downpipe part number 11999-KM002. It doesnt bolt onto my stock o2 housing. I have even tried to remove the donut gasket and it still does not fit. I measured the id of the downpipe at about 2.5-2.7 inches.

 

FYI the DP and 02 housing are never going to seat flush with each other. You need that donut gasket in there.
As long as the DP fits that donut gasket your gravy.