- My model of MX-5 is: __ NC 2.0
I’ve just failed an MOT due to the gasket between the rear box and exhaust breaking down.
I’m assuming that this is just a generic part - if does anyone know the dimensions?
A crush gasket…
Goes over the spigot on the flange on the mid pipe and should crush against same on the back box. I’ve just renewed mine.
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Best of luck with the nuts. Mine appeared to have been factory welded. Although the nut had rotted over 10/11 years the weld was stubbornly as new.
So I have a Cobra back box, and they have supplied this 3" gasket.
However, I’m still getting an exhaust gas leak, and can’t really tighten the bolts much more.
Would it be okay to use some Exhaust Assembly Paste?
Can you still get gun gum in the UK? c.1970 my exhausts were all held together with it. 
David
I would get the crush gasket in the link above. I had one of those flat gaskets that came with a Cobalt back box. After trying twice to seal the joint on my previous NC including some exhaust paste it didn’t really seal it. The crush gasket did the job.
I’m not sure there is anything for the crush gasket to crush against. The opening in the back box seems quite large.
Exhaust gum will be fine
Gun gum is ok but may flake or disintegrate if there is any movement of the flanges. The copper high temp rtv sealants have worked well in a similar situation for me with a Goodwin back box that has an oversized flange and flat gasket.