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MG MGB GT V8 Factory Originals Technical - Genuine exhaust manifolds

I've just swopped a surplus pair of quaterlight flames for a pair of genuine V8 exhaust manifolds. The intention was to use them as a pattern to remake out of cast MALLEABLE iron pipe bends to duplicate the original. The idea of malleable iron is that (a) the bends are tighter than when bent on a bender (b) it can be welded.
Looking at the ones I have they have a lug broken off the lower flange but are otherwie in reasonable condition.
I know everybody will say "buy tubular" but my question is - where do the originals fail other than the lower flange. What I'm not thinking is welding a piece of cast iron (I KNOW it's specialist welding !) at the top of the manifold where the ports all jopin into the one large pipe after cutting theoriginal off. The large pipe would then be duplicated in stainless steel thus only part of the original used. Do the head mounting flanges fail or crack ?
Thanks.
Paul Humphries

Paul,
I have a pair as well with a very thin section just above the flange on one. Curiously enough, the mate suffers no thinning and the metal is quite thick in all areas. I've often wondered how this thinning occured and if repairs might satisfactorily be made. This set has changed hands a few times because of the flaw.
Regards,
David
David

I've seen it writ that the flanges crack due to overtightening. The gasket being the shape it is there is a greater volume of gasket material between the bolt holes than under them. When tightened down the gasket under the bolts compresses more than between the bolts, which bows the flange. You can see exactly the same on a thermostat housing by putting a straight-edge across the holes. Trouble is the cast iron doesn't bend like the alloy or steel housing can. You can flat the thermostat housing on wet and dry on glass, BTW.
Paul Hunt

Has anyone within, say 50 mile of Stoke-on-Trent, a vehicle with the engine in and original cast iron manifolds fitted that I can look at ?
I will probably remake all of the manifolds out of tube but I'm intriged by the prospect of saving at least part of the originals. Another idea which has occured to me is cutting the original lower flange off and welding part of a cast iron manifold, off another car, on higher up hence wanting to look at a set on a car with engine is situ. I know others will have looked at this problem before but I need to satify myself that the originals aren't worth replacing / modifying before enbarking on fabricating from scratch.
Thanks.
Paul Humphries

Paul

I've just got to chip in with this. I'm a bit of an originality fiend on the quiet, but I can't help feeling that you're wasting your time with this. It's usually the downpipe mounting flange which cracks, and sometimes a large section of the pipe will come away with the flange section. They also sometimes crack at the head flange, but that's nowhere near so common. The other usual place is in the centre of the 'V'. There were two specs of manifold, and I recall that they were beefed up in later production. The early thin flange version was obviously inadequate - you only had to look at it. The other issues are that the cast manifolds are not as efficient as properly made tubulars, but note also that there are some awful tubulars around, with serious amounts of weld inside to restrict airflow. Finally, the access to the head/ flange bolts is tighter with cast manifolds, and you won't get the starter off without removing the manifold first. With some tubulars you can.
A good guide for tubulars is that you don't want to see a series of short shaped curved pipes welded together for each cylinder. The best will have a single curved pipe for each. Also, I believe its the third cylinder back on the offside, some tubulars are very bodged around the mounting bolt flange with almost impossible access. The solution is a decent set of tubulars (eg Janspeed) with allen head bolts.

Dave
Dave Wellings

Why not cast them out 4130 steel or 3 series stainless? Not much difference in price & both materials have better qualities than cast iron!

Dwight
Dwight McClain

This thread was discussed between 12/12/2002 and 15/12/2002

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