Buick 455 intake manifold 1973 plugs?

Hi everyone,

I own a 1973 Buick Centurion 455 and I wonder what are those plugs resp. holes inside the intake manifold just underneath the carburetor for? I've attached a photo from the internet of a 1971 manifold, cause in my intake manifold aren't those plugs. It seems like there were holes, partly congested by carbon, in my one. I'll attach a photo of mine later.

Can anyone explain, what's the benefit of those plugs or in my case of those no-plugs? Does it depend on the 1973' AGR System? Or do I just have a shitty problem with missing plugs from manufacturing? :-D

Thank you guys,
Heidelberg, Germany


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Active Member
Thats a totally pre EGR intake, like a 1970. After that in 1971 IIRC, the EGR style manifold has a "pipe" cast into the manifold to receive and distribute cold air from the A.I.R. compressor. The blocks are pretty much the same, but with the introduction of RGR, the heads were redesigned to match the intake.

The 1970 heads and intake on mine were intentionally plugged to keep the bottom of the carb cooler, but it runs so poorly cold that I am going to pull the intake and remove the plugs.

Is the yellow circle your reference? Notice the AIR is blocked off (by the arrow) on the 1971 and later intakes. The choke thermostat housing is also different. Wire brush the part number on your intake and refer to "Buick by the Numbers" easily found on google. Bill






Active Member
I understand you are from Germany so there is somewhat of a language barrier. Not sure exactly what you are talking about. Try to post a picture of your intake. There are several emissions systems including EGR (Exhaust Gas Recirculation), AIR, (Air Injection Reactor), and then there is the Exhaust crossover passage that runs under the carburetor plenum to warm the throttle body. Bill posted pictures of all three systems. I am unclear what holes you are referring to though.
Thanks for your answers by now. Larry is right, when I wrote 'AGR' I totally meant the EGR (in German exhaust means "Auspuff", so there comes the A from)

So back to my intake manifold: I attached three pictures of my one. In the first you see the cut off A.I.R system with the blue circle. Seems like someone of the pre-owners worked in the heating industry..
In the same picture I marked with the yellow arrows the point of my question: what are those holes or bores underneath the primary throttles for? When I took the photos I just realized that under my secondaries are plugs as well as in the "pre-1973" manifolds but under my primaries aren't some plugs as instead to some older manifolds which I compared on ebay.

In the other three pictures you see the bores under the primaries in detail. The left one (standing in front of the car) is kind of smaller than the right one. The left one is also much more congested by carbon (which may be reasoned by the connection of the PCV valve to the left side of the carburetor).


So I'd just like to know:
Why does my 73er intake manifold have those bores resp. doesn't have the plugs underneath the primaries and is it correct?

And if it's correct, should I try to remove the carbon sedimentation/deposition from the bore especially under the left primary?

Sorry if I repeat myself in some facts or if there were some unusual choice of words like carbon sedimentation. I'm not sure if I got the right translation 🙃


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I have never seen anything like that. As far as I know, those holes should not be there. Those holes go into the exhaust crossover? That would create a vacuum leak.
Interestingly, I just got a photo of an 1973 Pontiac 455 manifold from an ebay seller whereupon you can see some bores under the primaries, too.

My next step would be testing the bores with pressed air and see what happens. I guess for borescoping the holes are too small. And alternatively pulling the manifold.

But I'm a little bit scared. Your proposal with the vacuum leak would explain why I'm measuring only ~9" HG. I did this some months ago.


Active Member
Yo Bob... now Im confused. That appears to be a 430 or early 455 intake. NON-EGR equipped. That would mate up to the small heat cross over holes in the heads for plenum heating.
Blaue Reithauben has a manifold that is really restricted on the primaries, an A.I.R. port and an EGR valve. Was that possibly a European import mandate? Heads also have that giant crossover port. Never seen an intake like that!

With those primary restrictions, the vacuum should almost reach absolute! Curious about the carbon in there too. It does in fact look like its penetrated into the heat cross over. Bill
Yes, I agree, guess I should have looked at the picture. I will have to delete it and watch for another....

What is the casting number on the manifold in question, I thought it was 1231718? On a closer look, it appears to be 1243020
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The holes beneath the primary bores connect to the output port of the EGR valve. The Buick 455 and 350 first used the EGR valve on '72 California cars, then on all cars in '73. So that would explain why earlier examples shown from the Internet do not have holes beneath the primary bores.

It was a routine maintenance item to check the EGR valve operation and clean the manifold passages. If the valve is stuck partially open, that could cause your poor vacuum readings at idle.

That '73 intake manifold has generated questions in other posts in the forums here. It is a very busy piece of cast iron for sure. See attached pages from the '73 Chassis Service Manual for additional details.


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Hi Bob, thanks for your link. But that's only the cast of the 1970' intake from the internet I posted for comparison. The number of my 1973' intake is 1243020 and it's unfortunately not linked with an image on the reference page.
Thanks Todd for those pages! That's plausibel. I'll need some minutes to go through it but I'm really happy. I have owned the car since 2016, but I managed it mostly with the manual for the 1971 chassis from teambuick.com, so I've never had a description of the EGR or the AIR system. Thanks a lot! I also didn't check the AGR valve when I rebuilt the carburetor last year but I still have a new AGR valve around here. I'll change it the next days.
I guess you have the 73' manual as paper work, don't you?

By the way, is my nickname really shown as "blaue Reithauben"?! :D yachtmansbill called me like that in his post.
Yes I have a paper copy. It is quite worn, but invaluable for answering questions here. If you can acquire a copy, it will be worth its weight in gold when making repairs. A '73 Fisher Body Manual will be useful as well.

Yes the nickname appears in German in his post as you describe. It is in English at the top of each post of yours however.


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