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MGB Exhaust Manifold Differences

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tenntony Avatar
tenntony Silver Member Tony Shoviak
Waterville, OH, USA   USA
1952 MG TD
1958 MG MGA 1500 "Wife's Car"
1966 MG MGB
1969 MG MGC GT
What are the differences ,if any, between a 1962-1970 12H709 exhaust manifold and a 1971 -1974 1/2 12h3911 exhaust manifold?
Can a later 12H3911 be used on a 1966 GB engine? Thanks, Tony



Edited 1 time(s). Last edit at 2018-05-15 09:44 PM by tenntony.

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ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
I don't know the years, but the early manifolds, when twin SUs were used had separate inlet and exhaust manifolds. The exhaust manifold was considered to be as good as extractors, in road usage.

The later ones were one piece inlet and exhaust manifolds, to cater for the single ZS carby. The centre exhaust part was "cramped" to allow fitting of the carb, so the two inner cylinders had a lot more back pressure.

Herb



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tenntony Avatar
tenntony Silver Member Tony Shoviak
Waterville, OH, USA   USA
1952 MG TD
1958 MG MGA 1500 "Wife's Car"
1966 MG MGB
1969 MG MGC GT
Herb,
I'm sorry I was referring to the later manifold that still used the twin SU 's. Which was used until 1974 1/2 models. Thanks for your information. Tony

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mowog1 Avatar
mowog1 Gold Member Rick Ingram
Saint Joseph, IL, USA   USA
1969 MG MGC "Vicky"
1972 MG MGB "Mallard"
1974 MG MGB GT V8 Conversion "The V8"
1978 MG MGB "Maggie"    & more
There were two different flange thicknesses used on the twin SU cars.

I believe either can be used, but you need to match the thickness of the flange of the exhaust manifold to the thickness of the flange of the intake manifold.

This can be done either with matching pairs or by using washers cut in half as spacers to make up the difference.



1969 MGC - 1972 MGB - 1974&1/2 MGB/GT V8 conversion - 1978 MGB

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oleanderjoe Avatar
oleanderjoe Gold Member Joseph Baba
Fresno, CA, USA   USA
CORRECTOMUNDO. Last month somebody put up a picture of the OFF SET Manifold washers. .?? MOSS ?? smileys with beer


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dickmoritz Avatar
dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
Intake and exhaust manifolds originally fitted with HS4 carbs had mounting flanges that are 9/16 thick. Intake and exhaust manifolds originally fitted with HIF carbs had flanges that are 7/16 thick. The difference is just an eighth of an inch. The difference can easily be accommodated by gluing or welding half of a flat washer to a whole flat washer. I made up something a little more elegant for my race car...

With aftermarket headers you never know what flange thickness you're going to get...

Dick



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(Often wrong, but always certain)


Attachments:
MGB manifold washers.JPG    36.1 KB
MGB manifold washers.JPG

78 B PaceCar Avatar
78 B PaceCar George Funderburke
Kennesaw, GA, USA   USA
Dick, I like your washers. If you don't bond or glue the half washers to the large flat washer, it can be challenging to get the half washers to stay in place while tightening. I guess my fingers are just to big.....

oleanderjoe Avatar
oleanderjoe Gold Member Joseph Baba
Fresno, CA, USA   USA
DICKEYDIDIT: That's the ones I was remembering. pretty nice.


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jjonns74 Avatar
jjonns74 Gold Member John Dailey
Indianapolis, IN, USA   USA
1974 MG MGB "Athena"
1974 MG MGB "Apollo"
In reply to # 3737322 by 78 B PaceCar Dick, I like your washers. If you don't bond or glue the half washers to the large flat washer, it can be challenging to get the half washers to stay in place while tightening. I guess my fingers are just to big.....

It looks like Dick took a 1/4" spacer and removed 1/8" of material from one half of each of them = one piece, no glue required.

RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Geez, you'd think that he'd polish them once in a while. They're getting a bit rusty. RAY

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dickmoritz Avatar
dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
That ain't rust. It's patina... smoking smiley

Dick



In reply to # 3737423 by RAY 67 TOURER Geez, you'd think that he'd polish them once in a while. They're getting a bit rusty. RAY



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tenntony Avatar
tenntony Silver Member Tony Shoviak
Waterville, OH, USA   USA
1952 MG TD
1958 MG MGA 1500 "Wife's Car"
1966 MG MGB
1969 MG MGC GT
Thanks Guys for all the responses. Now I know why my B had those half round spacers under the mounting washers when I took the motor apart twenty five years ago.
Tony

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RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Patina, my foot! RAY

balloonfoot Avatar
balloonfoot Gold Member Lloyd Faust
Southlake, TX, USA   USA
flange thickness isn't the only difference....see picture.



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Attachments:
IMG_1202.JPG    77 KB
IMG_1202.JPG

Diver648 Gold Member Warren Siringer
Tucson, AZ, USA   USA
1967 MG MGB
1967 MG MGB GT
1971 MG MGB GT
1974 MG MGB
Dick:

When are you going to make up a bunch of those? Many of us would really like to buy a set. Another Cottage Industry!

Warren

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