Advice on removing cat section of exhaust

[Jitsuka]

1. My model of MX-5 is: 04 NBFL SVT
2. I’m based near: Leeds
3. I’m looking for technical help or recommendations on: Removing the cat section of my exhaust

Evening all.
Plenty of cat theft around, so I’m getting ahead of the worry and removing it myself. I’ve been quoted £780 for the cat, (Platinum Group Metals Recycling ltd - Home | Facebook), and have lined up a stainless replacement for only £130, as I’m not bothered about it being a sports cat, (MAZDA MX5 1.8 11/00-10/05 Catalytic Converter MA6073T).
More interested in it continuing to pass an MOT and that £650 difference will come in handy for tidying up the old girl!

My trouble is, I’m not as uninhibited as those who go at the car with an angle grinder as I quite like it and want to continue to enjoy it afterwards. As a result, I’m struggling with various partially concealed nuts and bolts.

While I’ve made reasonable initial progress, having removed the bracing, some form of under tray and the four nuts holding both ends to the rest of the pipe, I’m stuck with what appears to be a bracket from the pipe to the side of the gearbox.

What would happen if I went at the two larger bolts into the gearbox? Bad things?

The single, small nut on the back of the gearbox bracket looks like it would normally be the one to try to remove but, as it’s been baked by the heat from the cat for 15 years, it’s in bad shape! It’s also quite awkward to get a socket over it…

Next question is how to remove what I think is the O2 sensor attached with a lead to the side of the pipe. It’s quite awkward to get at! Is that easier to leave until the pipe is removed, then go at it?

Also, are the two bolts at the rear end of the section captive, or should I be able to remove them? The nuts have come off alright with a bit of effort, but the bolts seem fixed. Isatright?

Finally, do I need to remove, or simply loosen, the centre pipe and silencer in order to free up the cat section?

1. How to remove pipes from gearbox brace?
2. How to remove O2 sensor as it’s awkwardly placed?
3. Are the bolts captive on the rear of this section?
4. How much exhaust system do I need to remove, to get this section out?

[Jitsuka]

Anyone help a chap out?

[Scottishfiver]

To be candid, and polite as circumstances dictate, it’s had 95 views and no takers.
I’m left to guess how many possibly thought…take it to a decent exhaust fitter and do a “Pictures of the Queen” folding stuff deal tbh. Theres a lot of rusty bolt etc headaches there. Certainly best ramped in the air really.

[Jitsuka]

Yep. Shame really as there must be dozens in the group who’ve replaced their own exhausts, each of whom have gone through this step.

[Bren15]

Hi Jitsuka, thanks for the info on PGMR, I’ll have a look at them on Facebook.

I took the CAT off my 2004 NBFL and it looked just like yours. The access to the bolts was tight but possible. The two bolts at the rear of the gearbox came out without issue and nothing moved/fell off/came out, etc. IIRC they may be different lengths so worth noting which goes where. I expected a struggle from the crusty nut on the bracket but it did come off once the CAT was removed (and after lots of WD40). The bolts at the rear of the CAT section were captive on mine.

For the O2 sensor, I first disconnected the sensor plug inside the car next to the passenger seat rail (had to take the seat out and lift a bit of carpet as I couldn’t disconnect the plug!) and then removed it from the CAT once it was off the car. The sensor came out quite easily. Treat it gently (I understand they don’t like to be cleaned with brake cleaner or similar!) and it should work again fine.

I replaced the whole exhaust so it all came off at the same time but I seem to remember there was probably enough movement in the rubber hangers to get the CAT off without removing the rear section.

Whilst I took the exhaust off myself, a proper mechanic put the new one on so I can’t help with that part I’m afraid plus it was also on a 2-post ramp which was a massive bonus!

Good luck!

[Jitsuka]

You have my heartfelt thanks.

[Bren15]

You are very welcome and I hope it helps although I realise that I had described removal of the post-CAT O2 sensor, which only needs to come out if you remove the middle section of the exhaust.

You will definitely need to remove the pre-CAT sensor, which has to be disconnected at the rear of the cam cover.

[Jitsuka]

Quick follow up for the benefit of anyone else who’s interested.
The observant of you will have noticed that I said I’d removed both nuts at each end, though the header end has three nuts… I only found that this evening after having spent notable amounts of my weekly swearing and grunting allowance on removing the bolts holding a bracket to the bottom of the gearbox–to find that it’s not actually attached to the exhaust any more!
Having reached that point, however, I had to try to work out why the bleddy thing didn’t move and found this awkward looking sod.

With its position relative to the subframe, there was no way I’d get a socket over it and I started to wonder whether I’d have to remove the headers and everything just to get to it. While faffing around assessing options, I made this pleasant discovery.

The section is now free, though I have to try to get the precat O2 sensor loosened off which is proving quite awkward. A lot closer but still a way to go and Bren’s advice is therefore quite timely! All I have to do now is work out the cabling route to free it up once disconnected from the back of the cam cover and it’ll be out.
Finally!

It’s all useful learning and experience and, as my dad often told me, nothing worthwhile is ever easy.