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MG MGB Technical - Exhaust problems need help!

I have a 77B with catalytic converter. About 2 months ago the converter "cracked" at the flange that mounts directly onto the exhaust manifold. I replaced the converter with a "test pipe" instead of a new converter being that I am in a non emissions state. Within a short time the gasket between the exhaust manifold and the flange on the test pipe blew out. I re-did the job put in a new gasket which has held up with no problems but now the test pipe is "cracked" in the same spot as the converter did, at the flange that mounts to the manifold. The new pipe has less than 1000 miles on it. I am at wits end. Am I doing something wrong in the installation? I mount the test pipe with gasket firmly to the exhaust manifold first, then mount the front end pipe to the test pipe with it's gasket. Then secure the rest of the exhaust system to the brackets from front to rear of the car. Everything is tight and there is no play or slip. At this point I will either have to get a new converter or another test pipe and start again. Any thoughts you have would be greatly appreciated, thanks.
JCH Hibbard

My guess is that you have a mounting problem somewhere that is causing the joint to fatigue and fail. It could be engine mounts, gearbox mount or the exhaust itself. Is there any strain in the system when fitting it or is there any excessive movement when running. Get someone to blip the throttle while you look underneath to see what is happening. Could be tricky to sort out.

John
John Tampkins

Normaly there is a subport for the downpipe fitted at the joint of the engine backplate and the gearbox bell housing, fixing the exhaust system there.
It is intended to take any movement out of the front part of the exhaust system, where it is bolted to the manifold.
Ceck if it is not missing on your car. If fallen off, the downpipe will allways crack at the joint to the manifold.

Hope this helps

Ralph
Ralph

Thanks John and Ralph for the comments. All the brackets are present and in place from the bell housing to the back. Everything is tight and there seems to be no slip or play when running. I'm going to take at closer look at engine mounts today but I believe they are still within an acceptable range of use. I thought maybe there was a sequence so to speak for the connection of all these parts together? I'm puzzled.
JCH Hibbard

I think your problem falls into one of three areas: You have too much engine movement (bad motor mounts), the exhaust is not aligned properly, or your exhaust mounts are too rigid.

If your motor mounts look suspect at all, replace them. This includes the ones on the transmission. Often I see these as looking like black melted marshmallows!

When redoing the test pipe next time, drop the entire exhaust. Work from the engine first and finish at the tailpipe. If you have to use an excessive amount of force to get the exhaust mounts to line/mount up, you've found your problem. Since you are having breakage problems, make sure your strap mount that connects to the back of the engine/transmission is there AND properly aligned!

I've seen some cars with the exhausts "done up tight" using solid brackets. This can lead to fractured pipes. After much experience with exhausts I've come to the conclusion that the factory exhaust mountings are the simplest, most cost effective and easiest to use in the long run. They usually hold up well as long as the exhaust is properly aligned.

BTW - If you can find someone that can weld, you can repair the test pipe rather than buy a new one.

Alternately, for 1975 only, the exhaust "head pipe" eliminates the cat and connects to the rest of the exhaust.

Tom
Tom Sotomayor

Tom thanks for the comments. To riigid ehaust mounts...hmmm I need to take a closer look there. There is absolutely no play whatsoever in the exhaust system and with normal engine movement upon acceleration maybe that could just be where the problem is. I'll take a closer look this weekend. Thanks.
JCH Hibbard

This thread was discussed between 14/06/2008 and 20/06/2008

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