Catalytic Converter Threads Too Tight

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.

On Fri, 24 Dec 2004, Blake wrote:

> 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.

Are you the same guy who was on here a coupla months ago wondering why the
converter was empty?

> 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.

Have you considered the possibility your studs are slightly too large?

That kinda sounds like a standard to metric thread issue....

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

Blake wrote:
>
> 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.

Drill out the threads, nut & bolt 'em, have a nice day. Aren't there
any REAL mechanics out there anymore???