In theory, a dremmel and a torch should never be necessary for this job. I've removed more than two dozen downpipes (w/cats or otherwise) from mkiv's (most of which I did all by my lonesome), and I've only lost two nuts (both of which were removed wth a long chisel and a large hammer). I consider both nuts that I lost to be a learning experience so that I could come up with the following process:
Do not skimp on tools for this job!!! The trick is to use a deep-well 6-sided 14mm 1/2"-drive impact socket, a 1/2"-drive wobbly extension, a 2' breaker bar, and some
PB Blaster (not in that order, of course). Do this job by hand (not under any circumstances with an impact wrench!).
1) Get a very, very bright light on the nuts (you need 100% clear view of them). Now soak the nuts with PB blaster, and wait 1 hour.
2) Use the wobble in the wobbly extension to get that 14mm socket dead-perfect-square on the nut, with as direct a line as possible on the extension to the breaker bar. Do not turn the breaker bar yet (read step 2a first).
2a) If the nut is very, very badly rusted to where the 14mm socket is loose on the nut, go no further! Remove the 14mm, and pound a 13mm deepwell 6-sided 1/2"-drive impact socket onto the nut (with a big hammer, extension, and whatever else you need). Again, this 13mm socket must be pounded on
before you even try to loosen the nut (even just for fun - a very rusted downpipe nut
will not come off with a 14mm socket). Pound the 13mm socket on until it is completely over the nut.
3) Apply a smooth, steadily increasing force on the breaker bar - use a pipe if necessary......while watching the socket like a hawk to ensure the socket doesn't move off dead-perfect square on the nut.
This process sounds like a real f'in pain in the @ss, but it's about 100 times easier than dremmelling and torching with no space to work. Guys, trust me. With the right tools, 99.9% of the time this job can be accomplished 1-2-3.
If you do hit that unlucky .1%, dig out the dremmel, torch, chisel, hammer, band-aids, and little elfs to work in that
[email protected] space. Seriously though, follow the process above religiously and let me know if it doesn't work...for what it's worth, I sincerely doubt I'll be getting any messages.