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Old 03-15-2017, 11:49 AM   #1
BILT4ME
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Correct Factory Exhaust Manifolds 70 GMC K2500

I am looking for the CORRECT part numbers for both exhaust manifolds for a 1970 GMC K2500 3/4T 4x4, with a 350 and 4sp.

Both manifolds that are on this truck are broken and I am quite sure they have been replaced multiple times due to the VERY poor care that was taken of it. It was used in a construction business and was flat-out abused.

I am looking to start out as if I am going to restore to correct factory specification, then my wallet will help me decide from there.

I have searched this forum as well as many others and I cannot seem to locate the correct literature that I KNOW is out there.

I am looking for the CORRECT casting numbers for both left and right.

The CURRENT ones I have are:
3747038 (Left)
3932465 (right)

I believe the right one to be correct, but not the left one. Please help.
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Old 03-17-2017, 08:24 PM   #2
Ol Blue K20
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Re: Correct Factory Exhaust Manifolds 70 GMC K2500

I'm not sure on the part numbers but Dormin remakes them. They're ramhorns with straight dumps.
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Old 03-17-2017, 11:58 PM   #3
El Dorado Jim
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Re: Correct Factory Exhaust Manifolds 70 GMC K2500

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Old 03-20-2017, 01:17 PM   #4
BILT4ME
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Re: Correct Factory Exhaust Manifolds 70 GMC K2500

Quote:
Originally Posted by Ol Blue K20 View Post
I'm not sure on the part numbers but Dormin remakes them. They're ramhorns with straight dumps.
Yes, but the Dorman quality has suffered severely in the last few years. The ID of the standard ones are smaller ID and they ALL include the A.I.R. ports and I don't want that.

I am looking to install CORRECT manifolds for this truck, with the exception that I am eliminating the heat riser on the bottom of the manifold. I do not know which side originally had the the heat riser, as the truck that we have has been cobbled so much, I am not sure what is correct and what is not.

I want to be able to make this a pseudo-restoration, as we have most all matching numbers for the main parts and the under-hood paint is still original and the cab/front is in decent shape.

We'll start there, then performance modify later.

Thanks!
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Old 04-05-2017, 12:18 AM   #5
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Re: Correct Factory Exhaust Manifolds 70 GMC K2500

The heat riser is suposed to be on the passenger side. The dorman ramshorns do suck. I had a cracked manifold and bought a dorman replacement and it didn't fit well. The machining was so poor i couldn't get the center manifold bolts torqued down until i opened them up with my dremel. The bolt head was hiting the side of the manifold where they didnt machine enough metal away. And yes i had to plug the smog tube holes. Surprisingly i got another manifold from LMC and it fit well and didnt have any of the problems the doorman one did. I have no idea who manufactured it.

I guess you need to keep an eye the parts board or ebay for a straight dump ramshorn.
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Old 04-05-2017, 02:17 PM   #6
BILT4ME
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Re: Correct Factory Exhaust Manifolds 70 GMC K2500

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Originally Posted by seattle gmc View Post
The heat riser is suposed to be on the passenger side. The dorman ramshorns do suck. I had a cracked manifold and bought a dorman replacement and it didn't fit well. The machining was so poor i couldn't get the center manifold bolts torqued down until i opened them up with my dremel. The bolt head was hiting the side of the manifold where they didnt machine enough metal away. And yes i had to plug the smog tube holes. Surprisingly i got another manifold from LMC and it fit well and didnt have any of the problems the doorman one did. I have no idea who manufactured it.

I guess you need to keep an eye the parts board or ebay for a straight dump ramshorn.
The manifolds from LMC are the Dorman ones.

I have been looking on eBay for originals, but I cannot determine exactly which version is "correct" and I cannot confirm if different casting numbers have the same size outlet/dimensions/features.

There are at least 6 casting numbers that MAY work. I have also found that certain years of Corvette ones are actually 2-1/2" outlet instead of the standard 1.9" or 2" outlet.

I have found a listing of every casting number and what it "goes to", but there are no pictures associated. So, if I had all the time in the world, I would cross reference this list, then use eBay to find pictures, but I still would not have confirmed dimensions unless I actually PURCHASED a few. When they are going for $45-$100 for ONE manifold, that's not going to happen.

I find it interesting that GM changed the casting number every two years, with little evidence as to what changes were made. The fact that it has a new casting number means it is NOT the same as another one. It COULD be just a superficial change, but at the cost of casting molds, It would be poor planning to change the castings that often.

Thanks for your input!

My original question till stands. What are the CORRECT straight dump manifold castings for a 1970 GMC K2500 with a 350 and 4 speed?
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