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MG MGB Technical - Exhaust Donuts

I am replacing the exhaust donuts that seal the pipes against the manifold. The donuts have a metallic covering with a taper on one side, and an uncovered fiber side. Both the pipe and the manifold have a taper. Do you fit the metal covered side of the donut up into the manifold, or do you put the soft fiber side fit up into the manifold? Don't want do to it twice!
Joe Dufresne

The metal side goes toward the exhaust manifold. RAY
rjm RAY

The metal side goes down. The metal is only there as a support for the soft material, which is what does the sealing. On the downpipe there is a flange where the metal support rests. When you tighten the assembly, the gasket is compressed and the soft material is pressed against the manifold and squeezed tight around the downpipe tube. This effectively seals off the assembly and prevents hot gasses from escaping.
Steve S

Well, without checking further and going from memory, I had a 50/50 chance of being correct. Figures, I was wrong. Thanks, Steve, for catching it. RAY
rjm RAY

Thanks Steve, a local mg specialist also sent me the same advice as you did today, I had the metal side down previously, but it had been many years since I last dropped the exhaust and wasn't sure I had it correct last time. Turns out I did.
Joe Dufresne

Ray, I suspect you were thinking of the gasket between manifolds and head. People install those both ways for different reasons, but one of them is that the metal side goes toward the hotter of the two, which in that case is the manifold.
Steve S

Steve, you're correct, as usual. I haven't had my exhaust manifold disconnected from my stainless exhaust system in over 20 years. Since I use bolts, instead of studs, in the end holes of the exhaust manifold, I can drop the whole assembly if I have to pull the head. The last B exhaust system that I installed was on a '72 GT. The owner went with the Moss Tourist Trophy system, which uses flat gaskets in place of the doughnuts. A very loud system. RAY
rjm RAY

Having been doing these since they were new, I think they go metal side up, and that is how I've always fitted them, but I can't swear that I am correct. I also did a lot of Jaguar work, and Jaguar used similar O-rings, but they have metal on both ends. Both cars originally had machined tapered seats on the pipes, so support here is not an issue. No comments on the trash repro pipes. If the soft side is against the manifold, it seems that sealing should be easier, but any leakage will blow the asbestos out, especially if the system is allowed to run loose, which is very common (and it is likely worse with our non-asbestos parts now). The metal prevents this, and the metal cap is quite soft and seals just fine.
Don't see any advantage to using bolts, I can drop the system either way; I see that as a retrograde move.
Do go back and retighten after some running, and make sure you have the steady brackets off the bottom of the bell housing, or you will be doing it again.

FRM
FR Millmore

With the exhaust manifold still connected to the down pipe, trying to remove them from the head will result in your hitting the frame rail and not being able to clear the end studs in the head. By replacing the end studs, with bolts, you don't have to pull the exhaust manifold as far and it will easily drop down, out of the way, to allow easy removal of the head. RAY
rjm RAY

Actually, Damn good idea Ray. My system was 'welded' by the exhaust man, not my idea, since I wanted to be able to dismantle it for experimenting later. Still bolts at the front end would be a help. So would these be mild steel bolts or brass? Allen headed? Mike
J.M. Doust

I used a pair of stainless steel bolts to hold the ends of the exhaust manifold in place. They've been in on the car for over 20 years and have been removed a few times, once to install the supercharger and once to replace the head gasket along with a set of ARP head studs. They work very well and have given me no problems at all. RAY
rjm RAY

This thread was discussed between 23/10/2011 and 29/10/2011

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