I took pictures of my 4 year old Cobalt removed from my last NC. Annoyingly I can’t find the pics ATM but looking at it after cleaning up it looked pretty decent. The only real rust evident was around the hangers and associated weld points. The pipes coming from the main box were discoloured, main box part cleaned up to a nice shiny finish again.
I had to take mine to a local back street place and have some heat applied to the nuts. Cost me £20
The one I removed from my 58 plate car was a ■■■■■■. Broke a dozen drill bits getting it off. Latest one, 13 plate, all over in half hour. Hammered a 13mm socket on and off they came. There is good access to the nuts if you take the bumper off but that’s a bit severe.
Hi, are these still MOT friendly?
Thanks
If you mean the Gravity performance back box - yes - no probs at all.
Technically it is just a replacement back box - a bit louder - yes - but not so an MOT tester would take note.
Thanks, the decat down pipes I was wondering about. With no cat how do people get their cars through an MOT without changing back again??
Thanks again
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The stock back box on my NC took some persuading to remove. The nuts were crumbling using nut extractors and I resorted to sliding in a hacksaw and cutting off the studs. Once removed, I drilled holes in the stud bases, refitted the back box with nuts and bolts and then daubed them with waxoyl.
A few years later when swapping over to the Cobalt, the nuts came off fine, I don’t think there’s too much heat in the back box to melt off the waxoyl.
The Cobalt has been on for over a year now and I’m really happy with it. It looks and sounds great. At UK legal motorway speeds it’s fine, but in Germany on the autobahn there’s quite a drone at 85mph plus. At first it gave me a headache but I got used to it.
If I take the GF over there I might have to revert to stock, it’ll make her ill.
I’ve not had mine MOT’d since so can’t comment fully. However, the main catalyst is in the mid pipe so you should be able to demonstrate compliance. I could be wrong but I think the cat in the downpipe is for emissions at low temp/cold start?
A couple of threads on here have clarified the rules around what passes/fails an MOT regarding removal of cat. I’m feeling confident it will pass though 
Ali
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There are two sensors - one on either side of the downpipe cat.
Have you moved the sensors so they are now either side of the mid pipe cat ?
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Hi, I’ve swapped the sensors round (top to bottom and bottom to top) but kept them on the downpipe. The new manifold had sensor mounts. It threw an EML but I’ll clear this with a remap.
Ali
Got photos of the gravity backbox fitted? Very tempted by it as I hate the look of the OEM exhaust.
I drive past another MK3 most days and the state of his aftermarket backbox terrifies me though. The whole thing is slanted so the tips are -45° from horizontal. Basically pointing at the floor.
Thanks guys, I think they look far better than OEM without going bean tin crazy.