I just bought a new "direct replacement" catalytic converter for my
Daihatsu. A thing that surprises me is that the bottom threaded holes are
slightly small in diameter.

It doesn't seem to be the "wrong" size. It should be 10mm x 1.25. The thread
pitch looks right, and the diameter is clearly closer to 10mm than to 9 or
ll. But it's just so tight that the studs won't even get started.

I could go buy a tap and cut the threads again, but I have to wonder whether
that's the right thing to do. Are the threads done this way for a reason? Is
there a procedure for expanding the flange with heat before installing the
studs? I wouldn't want to do the wrong thing and have my headpipe fall off
while driving.

Your advice is appreciated.