I replaced the catalytic converter on my 93 Miata with a test pipe. The issue is there is an exhaust leak that I can't seem to fix. I tightened the flange all the way around and it still leaks.

It only leaks from one side and at first it leaked from below the pipe but after flipping the gasket and re-tightening it now leaks from above. I tried to buy a new gasket and had the same issue. I spent more than an hour around that flange and tightened it as best as I could but it still leaked

Is it safe to use RTV to get rid of the leak? Otherwise how can I fix this problem?

Update: I used 2 gaskets and the leak improved but it was still there. One thing that came to mind is I didnt clean the mating surface of the pipe, is that something that should be done to exhaust flanges as it never occured to me?

If so how is it done properly?

  • 2
    RTV isn't going to cut it. Is this on the end with the donut or the one that goes to the midpipe? A better solution than RTV - which will work until it gets cooked by the heat of the exhaust - would be a thicker gasket. The most likely issue is that the flange on your test pipe isn't flat, so it won't mate with the gasket + exhaust manifold correctly.
    – 3Dave
    May 3, 2018 at 1:03
  • The gasket that came with the pipe is pretty thick, thicker than the one I bought from the store. Is it safe to double gasket? could that solve it? and the leak is not from the header side but the one facing the muffler
    – method
    May 3, 2018 at 1:13
  • 1
    I have had good luck sealing this type of leak with Muffler Putty. It is like a premixed refractory cement. I've only used the Loctite brand which comes in a can. Apply a liberal amount to both sides of the gasket. Tighten the flange to squeeze out the excess. I have had the best luck letting it dry overnight so the exhaust pressure doesn't blow it out.
    – mikes
    May 3, 2018 at 1:25
  • Are the flanges bent or mis-aligned?
    – Solar Mike
    May 3, 2018 at 5:44
  • @SolarMike No it seems perfect, I got it from racing beat
    – method
    May 3, 2018 at 6:06

2 Answers 2


Due to my own incompetency, @David Lively made it clear for me. I was not aware that the old catalytic converter gasket was baked on the mid pipe. Just running my hands on it made it so obvious.

Thank you all for the help.


enter image description here It's new. Look into how I fixed my leaks. C Flange Bracket

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