'71 GMC 2500 exhaust manifold (350) interchange.

[W. Higgins]

My left exhaust manifold is broken. I thought this was going to be a simple RockAuto thing. They don't list any left manifolds for '71 GMC 3/4 tons. I thought this was a simple oversight, so I searched the same for Chevy and get Dorman 674-201. Sure enough, on the interchange list this manifold fits every GMC over and below (1/2 ton and 1 ton) but not my 3/4. It looks like the manifold shown in the photo with the curved 3-bolt outlet. The Dorman site provides the same information. What am I missing here?

The only other clue I've found is this outfit lists a curved outlet manifold like Dorman's and like what I have on mine:

https://www.1aauto.com/chevy-gmc-exh...iABEgJIUPD_BwE


.... however, they also list a pair with a straight dump for the same application:

https://www.1aauto.com/chevy-gmc-exh...SABEgK3jPD_BwE


Am I getting bit by some kind of mid-year parts change where the books might not be up to snuff?

[LockDoc]

Originally Posted by W. Higgins

My left exhaust manifold is broken. I thought this was going to be a simple RockAuto thing. They don't list any left manifolds for '71 GMC 3/4 tons. I thought this was a simple oversight, so I searched the same for Chevy and get Dorman 674-201. Sure enough, on the interchange list this manifold fits every GMC over and below (1/2 ton and 1 ton) but not my 3/4. It looks like the manifold shown in the photo with the curved 3-bolt outlet. The Dorman site provides the same information. What am I missing here?

The only other clue I've found is this outfit lists a curved outlet manifold like Dorman's and like what I have on mine:

https://www.1aauto.com/chevy-gmc-exh...iABEgJIUPD_BwE


.... however, they also list a pair with a straight dump for the same application:

https://www.1aauto.com/chevy-gmc-exh...SABEgK3jPD_BwE


Am I getting bit by some kind of mid-year parts change where the books might not be up to snuff?

Can you post a picture of your manifold?

LockDoc

[W. Higgins]

I haven't removed it yet and the heat shield is still in place. I can take a shot now in that condition. I wasn't going to remove it until I had the replacement in-hand, but looking at it from underneath it looks like it doesn't much matter as it has crumbled at the bend. I'll go out in the dark and see what I can do.

[Andy4639]

The straight ones are for 4x4 trucks, the curved are for all the 2 wheel drives as far as I have ever herd.
Go back and look at the fitment chart the straight ones are all 4x4 blazers and such. They don't say it but the K10 is all that is listed no 2 wheel drives.

[W. Higgins]

Originally Posted by Andy4639

The straight ones are for 4x4 trucks, the curved are for all the 2 wheel drives as far as I have ever herd.
Go back and look at the fitment chart the straight ones are all 4x4 blazers and such. They don't say it but the K10 is all that is listed no 2 wheel drives.

Thanks, that's helpful. Mine is 2WD, so that rules out the straight ones, but it still doesn't explain why my 3/4 ton is the only one exempted from the application chart.

I tried to upload photos but am getting some kind of grief about a missing "security token". If I hear from an admin about how to resolve that I'll get them posted straight away.

[Stocker]

Originally Posted by W. Higgins

Sure enough, on the interchange list this manifold fits every GMC over and below (1/2 ton and 1 ton) but not my 3/4. It looks like the manifold shown in the photo with the curved 3-bolt outlet. The Dorman site provides the same information. What am I missing here?

The only other clue I've found is this outfit lists a curved outlet manifold like Dorman's and like what I have on mine:

As stated above, straight dumps are for 4x4s and curved dumps are for 2WD. As to listings that show 1/2 ton and 1 ton but not 3/4 ton..... well, that has to be an error in the listing. The engines are the same, and the manifolds will be the same also.

[W. Higgins]

Originally Posted by Stocker

As to listings that show 1/2 ton and 1 ton but not 3/4 ton..... well, that has to be an error in the listing. The engines are the same, and the manifolds will be the same also.

That's my thinking on the subject, too. At first I wrote it off as a RockAuto error because I've seen things that like before, but when I went to Dorman and found the same I wanted to make sure I wasn't making a mistake. Maybe it's that way on RA because Dorman supplied the information to them. Way back when I did the exhaust I don't remember there being any options for the Y-pipe, so I'd have to think they're all the same. The only thing I could think of, and it's kind of a stretch, is the new manifold shows plugs protruding for emissions connections where there are none on my original. It looks like the heads of these plugs would interfere with the heat stove, but I would think all the other models used this same heat stove on the left manifold, as well. I'm going to go ahead and order it so I can move on with it. I need to use my truck for something here soon and this was an unexpected surprise. When I started all this I just thought I was going to be replacing a donut gasket until this discovery!

[geezer#99]

Originally Posted by W. Higgins

Thanks, that's helpful. Mine is 2WD, so that rules out the straight ones, but it still doesn't explain why my 3/4 ton is the only one exempted from the application chart.

I tried to upload photos but am getting some kind of grief about a missing "security token". If I hear from an admin about how to resolve that I'll get them posted straight away.

Security token means your pics are too big.
Make them smaller.

[W. Higgins]

Originally Posted by geezer#99

Security token means your pics are too big.
Make them smaller.

Ah ha! I guess it would have been too easy for them to say it as you did. Thanks!

[aotte1]

These are photos of 1972’ 4WD exhaust manifolds, from a 350 Engine. Not sure if they help you, can compare yours to see if the same. They are correct for K10s, not sure for K20s.
3932461 is the casting number, same for both sides.

Les

[W. Higgins]

Thanks for those photos. These are mine, definitely curved, which fits the bill with my 2WD truck.

Laying underneath in the dark with a light I can now see that the elbow on the left one has crumbled and shattered like a piece of safety glass. I thought before placing my order that I'd see how many studs were going to snap off of the right side manifold and from below I can see the beginning of a crack and the same shatter lines, so it's not far behind and I ordered a pair.

Interestingly enough, Dorman does the same for the right manifold in that it omits my 3/4 ton only. When you go back to RockAuto, a right-side Dorman manifold isn't even an option under the same Chevy listings where I found the left, but if you manually search the Dorman part number it can be had. Just goes to show that it always pays to have the part number if you're intent on having something specific since even their own catalog is a little wonky.

[Andy4639]

I just finished cleaning these with a wire wheel and painted them with brake caliper black paint. Going on the 350 in my daughters 68 step side.

I'm wondering if they mean they fit a 1/2 ton up to a 1 ton.

[Stocker]

Originally Posted by Andy4639

I'm wondering if they mean they fit a 1/2 ton up to a 1 ton.

That never occurred to me..... but I bet you're exactly right!!

[randy500]

I have a real clean set of manifolds for sale on the parts forum....

[W. Higgins]

Originally Posted by Andy4639

I'm wondering if they mean they fit a 1/2 ton up to a 1 ton.

Nope, such is not the case. It's the application chart that's the issue and now looking at it in light of what was posted above about straight dump manifolds being for 4WD it would seem to be in error with respect to that, too. Also, on RA and Dorman when I used the "check fit" feature, it came back telling me that it would not:

CHEVROLET BLAZER 1969-1972
CHEVROLET C10 PICKUP 1969-1972
CHEVROLET C10 SUBURBAN 1969-1972
CHEVROLET C20 PICKUP 1969-1972
CHEVROLET C20 SUBURBAN 1969-1970
CHEVROLET C30 PICKUP 1969-1972
CHEVROLET G10 1970-1972
CHEVROLET G20 1970-1972
CHEVROLET G30 1971
CHEVROLET K10 PICKUP 1969-1972
CHEVROLET K10 SUBURBAN 1969-1972
CHEVROLET K20 PICKUP 1969-1970
CHEVROLET K20 SUBURBAN 1969-1970
CHEVROLET K30 PICKUP 1969-1972
CHEVROLET P20 1970
CHEVROLET P30 1970
GMC C15/C1500 PICKUP 1969-1972
GMC C15/C1500 SUBURBAN 1969-1972
GMC C25/C2500 PICKUP 1969-1970 -- this one is me.
GMC C25/C2500 SUBURBAN 1969-1970
GMC C35/C3500 PICKUP 1969-1972
GMC G15/G1500 VAN 1971-1972
GMC G25/G2500 VAN 1970-1972
GMC G35/G3500 VAN 1971
GMC JIMMY 1970-1972
GMC K15/K1500 PICKUP 1969-1972
GMC K15/K1500 SUBURBAN 1969-1972
GMC K25/K2500 PICKUP 1969-1970
GMC K25/K2500 SUBURBAN 1969-1970
GMC K35/K3500 PICKUP 1969-1972
GMC P25/P2500 VAN 1970
GMC P35/P3500 VAN 1970

[Andy4639]

Well what ever the chart say's aren't doesn't say the curved ones will fit your truck. They are the ones you need and they are only made 2 ways Either curved are straight.

[W. Higgins]

Glad to hear it because replacements are already on the way! What started this was replacing the tailpipe and muffler. While I had them loose I went to replace the donut gaskets because I had some noise up front. When I grabbed the pipe, it shook, and then I really had some noise! This would have to happen on the side that's totally concealed or else I would have seen it coming. Thanks to everyone for your input. I'm up against the clock to get it going again and needed to make a decision quick.

(Sorry for the rotation. I can't get it to behave.)

[Stocker]

Quick story that has nothing to do with exhaust manifolds but has something to do with application listings.

My '72 K20 with A/C came with a HD fan clutch. When it failed, I discovered the only listings were for light-duty units. I checked parts houses and the Hayden website and they all said the same thing.

Found a HD unit listed for a mid-Seventies K30, but the sites all said it would not fit my K20. I ordered it anyway. It was a perfect fit and works flawlessly.

[W. Higgins]

Along those same lines, and I share this hoping it will be useful to people seeking parts, I went through the same thing trying to source a muffler and pipes. Rock Auto had a couple of mufflers and the front pipe, but no tail pipe. I didn't want anything exotic, just standard stuff to get it blowing out the back again. If you go to Walker you can look up whole systems by application and it will show the part numbers for each component. A part number search on Rock Auto with the Walker number showed that they did have the tail pipe, but it's not where you can look it up by application.

[AussieinNC]

Originally Posted by W. Higgins

Glad to hear it because replacements are already on the way! What started this was replacing the tailpipe and muffler. While I had them loose I went to replace the donut gaskets because I had some noise up front. When I grabbed the pipe, it shook, and then I really had some noise! This would have to happen on the side that's totally concealed or else I would have seen it coming. Thanks to everyone for your input. I'm up against the clock to get it going again and needed to make a decision quick.

(Sorry for the rotation. I can't get it to behave.)

Make sure you use plenty of rust breaker spray on that stud and nut....they can really hang up after being on there a while...

[weim55]

Originally Posted by W. Higgins

Glad to hear it because replacements are already on the way! What started this was replacing the tailpipe and muffler. While I had them loose I went to replace the donut gaskets because I had some noise up front. When I grabbed the pipe, it shook, and then I really had some noise! This would have to happen on the side that's totally concealed or else I would have seen it coming. Thanks to everyone for your input. I'm up against the clock to get it going again and needed to make a decision quick.

(Sorry for the rotation. I can't get it to behave.)

Holy cow! You have a problem somewhere that caused those manifolds to fail like that. That is something that doesn’t just happen over time. Those manifolds were glowing cherry red from intense heat multiple times that caused them to fail. Once you get the new manifolds on you might want to look into a possible problem so the new ones don’t fail as well. Was the old muffler clogged up? Any symptoms of a poor running engine recently?

Steve weim55 Colorado

[W. Higgins]

Originally Posted by AussieinNC

Make sure you use plenty of rust breaker spray on that stud and nut....they can really hang up after being on there a while...

Yeah, that oddball bolt / stud nut came out in one piece fortunately. Not wanting to break it I heated and quenched it first and the nut came right off. I'm happy to report that I was able to work out all of the manifold bolts this morning without any of them breaking. Thanks for the note.

[W. Higgins]

Originally Posted by weim55

Holy cow! You have a problem somewhere that caused those manifolds to fail like that. That is something that doesn’t just happen over time. Those manifolds were glowing cherry red from intense heat multiple times that caused them to fail. Once you get the new manifolds on you might want to look into a possible problem so the new ones don’t fail as well. Was the old muffler clogged up? Any symptoms of a poor running engine recently?

Steve weim55 Colorado

That's my thought from the looks of it, too, but I've never had any issues. It has always run fantastic and still does. I bought it right before I signed up for this forum about eleven years ago. It had about 70,000 original miles then and has about 105,000 on it now. It doesn't eat plugs and the fuel mileage has always been consistent. I have done a lot of heavy towing with it in the 7,000 lb to 10,000 lb range including a 1,200 mile (one way) trip on the interstate when I first got it and has never given me a moments concern. When I moved a few years ago during several trips I put over 3,000 miles on it hitched up to a 24' enclosed trailer the whole time. I've used it when it's 10F out and also hopped right back in it after a coffee stop on 100F days hauling a heavy load and it always fires right up and idles without issue. The previous owner bought it new as a dedicated slide-in camper hauler and probably towed with it, as well. Since this is the first time the heat stove has been off I cannot tell you if there were any signs prior to my ownership.

[AussieinNC]

Originally Posted by W. Higgins

Yeah, that oddball bolt / stud nut came out in one piece fortunately. Not wanting to break it I heated and quenched it first and the nut came right off. I'm happy to report that I was able to work out all of the manifold bolts this morning without any of them breaking. Thanks for the note.

As weim55 mentioned....the muffler on this truck must be clogged up...these manifolds are usually bulletproof,,,to see these holes is strange to say the least...they crack...yes, they rust up the pipe to manifold studs...yes....but blow the side out of the manifold...WOW !

Check that muffler before you blow out the new manifolds....

[W. Higgins]

Originally Posted by AussieinNC

As weim55 mentioned....the muffler on this truck must be clogged up...these manifolds are usually bulletproof,,,to see these holes is strange to say the least...they crack...yes, they rust up the pipe to manifold studs...yes....but blow the side out of the manifold...WOW !

Check that muffler before you blow out the new manifolds....

As I mentioned above, I started this project to replace the muffler and tailpipe and I was in the middle of that task when I discovered the manifold. It was to replace a muffler that I put on it about six years ago and the thing has shown no signs of restriction nor a change in operation.