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MG MGB Technical - Manifold to Head - washer to head distance

So.. surprise surprise...

on 68 Mgb.

grabbed a different manifold to re-stud for exhaust repair, and low and behold on bolt up, the replacement exhaust manifold I grabbed had a 5mm thickness from washer bolt down to the head surface. The previous
manifold (that matched the intake manifold) had 5.7mm thickness. Which is standard.

SO.. what is the difference. Why the smaller gap.
Is this narrower gap manifold set, from race milling..
or.....?? All my other manifolds are the standard distance. This modification shortens the air path from the manifolds to the head.

appreciate the education... I don't recall noticing this in the past, I guess it was just blind luck.. so now to root around for the other intake manifold that matches this exhaust manifold.

Is this smaller air gap a good thing performance wise.. or no... I am using the HS4.? carb's from a '73. the ones with bottom bowls, not side bowls.
Jim Fink

Jim,

I'm not exactly sure I know what you are saying, but it sounds like maybe you got an exhaust manifold from a different year car.

There are thick flange and thin flange intake/exhaust manifold sets. You have two choices. Either you use a matched set, or you build up a set of staggered washers so that the nuts will clamp both manifolds where the washers span between the intake and exhaust.

I ran into this on one of my cars, and I built up a set of staggered washers. If this is your problem, just cut some washers in half and braze the halves to the full washers.

Charley
C R Huff

Jim. I, too, am not exactly sure of what you are asking about. It sounds like you are using an HIF intake manifold (fuel bowls integral with the carb body) which has a slightly thinner flange than the one originally used with the HS-4 carbs which came on your 68. The exhaust manifolds were produced with flanges the same thickness as the intake manifolds and, if possible, the two should match. If not, as Charley suggests, you will have to shim the thinner flange out to the thickness of the larger flange using washers or some other form of shim material.

Les
Les Bengtson

Jim-
The Original Equipment SU exhaust manifold used with the 1˝” SU HS4 Series carburetors’ SU intake manifolds (BMC Part # 12H 911, 12H 1397, 12H 2571, 12H 2575, and 8G 767) have a mounting flange thickness of 9/16” (14.2875mm) and can be readily identified by its external casting number of 12H 709 (BMC Part # 12H 709). On the other hand, the Original Equipment SU exhaust manifold used with the 1˝” SU HIF4 Series carburetors’ SU intake manifold (BMC Part # 8G 774 ) has a mounting flange thickness of 7/16” (11.1125mm) and can be readily identified by its external casting number of 12H 3911 (BMC Part # 12H 3911).
Stephen Strange

You are saying that there is a difference of .028" from this manifold to "standard"? Standard according to who/what? 5.7mm is 0.224", less than 1/4", which is much less than any MGB manifold ever was, are you sure it's not a midget? 0.028" is quite possibly within production tolerance for machining the casting, possibly it has been cut to make it flat after some yert bent it by over tightening. I just measured a thick flange manifold and the three relevant measurements range from 0.565 to 0.577 on the SAME manifold.

"This modification shortens the air path from the manifolds to the head" Can't argue with that, but I think that I am definitely now on some strange planet. Do you actually think that such an amount in exhaust path length could possibly make a difference in an engine? Must be something here I don't understand.

FRM
FR Millmore

Well, great stack of info all...
yes, well I was muddled on what gauge my micrometer was displaying, yes, a diff. of .570 and .500 or so,,,,

well that is why I asked...about the air flow diff.. I had no idea why there was that difference, and if it was a good thing or not. Guess it makes no real diff.

I have been using the HIF-4 Carburetor's and it looks like I just found the correct intake and exhaust manifolds too use.

Thanks all.. sorry I had misconstrued some of my facts, and questions. You guys guessed my inquiry correctly, hard to describe that measurement difference.

Glad to re-visit the Scions .. It's been a while, ya'll are great.

Jim
Jim Fink

This thread was discussed on 22/11/2009

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