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MG MGB Technical - Exhaust manifold/test pipe gasket blew out.

I recently had the weld break on my catalytic converter at the front flange where it attaches to the exhaust manifold. I replaced the converter with a test pipe as I am in a non emissions state. After completing all the work associated with the job, 10 miles up the road the gasket between the exhaust manifold and the front flange of the test pipe blew out. Everything was connected correctly I did not use any gasket dressing on the gasket itself under advisement. Has anyone ever had this gasket blow? It was of course a fresh one. Should I have used a dressing? I'm going to do the job over again this weekend and any thoughts would be appreciated.
JCH Hibbard

I don't know if this relates at all to your problem, but this happened to me when I first got my MGB.
Not exactly - but like it.

The downpipe was supposed to have a ring around the pipes, those were worn very badly so the connection could not be made tight. I had to get a new downpipe, then after installing it no more problems.
Rick Bastedo

Thanks Rick. No problems on the down pipe end. Everything is good there. The problem was up at the other end where normally the catalytic converter attaches to the exhaust manifold, it was that gasket in between the two that blew out. Or in this case between the manifold and test pipe. I tend to think that maybe I should have used some high temp gasket cement or maybe the flange on the test pipe was not perfectly flat ensuring a good connection. I don't know but I do know the gasket should not have blown out.
JCH Hibbard

JC. No gasket cement needed. If the new gasket blew:

You did not install it correctly

It was an inferior gasket

Something about the mating surfaces is incorrect and damaging the gasket when you are installing it correctly

Combination of the above.

I might note that the design of the rubber bumper (RB) combined intake/exhaust manifold is, at best, rather poor. The earlier type exhaust manifolds have the flanges pointing almost straight downwards, thus each stud on the flange is under nearly equal pressure and the gasket (donut, doughnut, dognut) between the manifold and header pipe is not stressed to one side as the later model system does.

I have had good luck using the Fel-Pro gaskets. I have assembled a couple of the RB engines, using the original exhaust systems, with the Fel-Pro donuts as the gasket and they have held up reasonably well. At least a year's use.

You might obtain a Fel-Pro donut, clean all of the mating surfaces, inspect them using a light and inspection mirror on the exhaust manifold mating surface, and use some anti-seize on the studs when clamping the flanges over the donut.

All of my cars are either running the earlier factory exhaust manifolds or headers. Poor design of the RB exhaust manifold and a tendency to crack on the inside of the radical bend made that a desirable modification, at least to me.

Les
Les Bengtson

No one's mentioned it, but there should be a clamp-type deal on the down pipe that attaches to the engine/trans somewhere (I forget) that stabilizes the exhaust system - preventing it from swinging and allowing the donut to blow out.

Wayne
Wayne Pearson

Les and Wayne thank you for your help. I just want to be sure we are talking about the same thing. I am referring to the gasket between the exhaust manifold and the catalytic converter (or in this case the test pipe I replaced the converter with). When you say donought are you referring to the flange gasket at the down pipe? Les I tend to agree with your idea on inferior gasket and mating services. Any further suggestion on how to correctly install this test pipe and gasket? I thought I had it perfect?
JCH Hibbard

I have a 75 Roadster that was not a California car, and consequently had no catalytic converter originally. There was a specific OEM exhaust pipe for that year that "bolted" directly to the intake/exhaust combo manifold, using a donut-style gasket between. As I recall the opening in the manifold has the edge tapered that accepts the donut gasket as well as the pipe being flared to accept the other side of the donut. Again, if I remember correctly, the donut I had was steel instead of composition (but it shouldn't matter). I assume that the exhaust manifold has enough surface for a flat gasket if the catalytic converter is used - I can't say since I've never seen one first hand.

Snug up this connection (with the rest of the exhaust system installed) but still able to wiggle about slightly. Anchor the exhaust pipe to a lower bolt on the bell housing using a steel strap and an exhaust clamp. Then finish tightening the manifold/exhaust pipe connection as evenly as possible.

Wayne
Wayne Pearson

JCH,

A friend recently had the same exact problem that you describe. It appeared that the supplier of the "Test Pipe" provided the wrong gasket for this application. Appeared to be more of a thick(er) paper water thermostat gasket than a high temp exhaust gasket. When he replaced the gasket with the correct stlye high temp gasket the problem went away completely. If you do not already have the heavy high temp I believe you can obtain it from Moss or possibly match it up with a gasket at your local muffler shop or NAPA dealer.

HTH

Fred
Fred Wright

This thread was discussed between 27/01/2008 and 31/01/2008

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