'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.

[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.

[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!

[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.

[randy500]

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

[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.

[BigBird05]

The only time I have seen a manifold turn cherry red was when it was run on a Dino at WOT and maximum load. That kind of sustained heat will do that to cast iron.

[W. Higgins]

Originally Posted by BigBird05

The only time I have seen a manifold turn cherry red was when it was run on a Dino at WOT and maximum load. That kind of sustained heat will do that to cast iron.

On earlier vehicles where you manually control the spark advance having the spark severely retarded will do it too, but in this case I haven't had the timing slip and when you run that heavily retarded you'll eventually see the coolant temp rise, which also hasn't ever been an issue.

[W. Higgins]

I swapped out the manifolds last night and had it running before bed. Took it out for a drive after tidying up some things this morning and it's still the same great running truck it always has been. While the manifolds were off I pulled the plugs and did a quick compression check. Everything is about 150lbs+. Timing is exactly where it's supposed to be and nothing is amiss. Thanks to everyone for helping me quickly resolve the part application issue so I could get this out of the way before the week started.

[AussieinNC]

Many thanks for the update!

The folk here love to help everyone out...

Enjoy your truck !

[W. Higgins]

I know how people like to see these old beasts still in service. This was back in the spring dragging home a new piece of iron.