1984 gas conversion exhaust manifold issues

[Jimmy84]

I'm in the process of replacing my 84 6.2l with a 1994 G20 5.7l TBI engine. The engine has less than 70k miles and is in great mechanical condition.
The problem I'm having is trying to connect the 84 diesel Y-Pipe to the 94 exhaust manifolds.
When I performed this conversion before, the diesel Y-Pipe interchanged with the donor 350 without issue.
On this Jimmy, the existing diesel y-pipe and flange bolt-hole spacing are smaller then what is required by the 94 exhaust manifolds. The 94 TBI manifolds have a 3" center to center flange- bolt spacing (see Pic) and the crown of the manifold outlet measures 63cm (2.48", see pic).
I searched the site but I couldn't find anything on this particular subject.
Has anyone else had this experience? Does anyone know when Chevy changed the outlet size of the manifold?
Don't want to purchase a brand new $180 Y-pipe if I don't have to.
I hate to change the manifolds because they are in really good shape. I was able to remove the original manifold studs with a small pipe wrench.

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Thank you

 

[Bextreme04]

I'm not surprised they don't fit. The GMT 400 has a wider frame rail. I am surprised that the reason they don't fit is the outlet diameter though, I would have expected frame interference to be an issue.

 

[Jimmy84]

The engine slipped right in just as you would expect. And the pipes are in line with the manifolds.
Engine mounts were completely different though. The through holes on the van mounts were wider, which goes to the point you made about frame rails.

 

[78C10BigTen]

If it was me... id just make custom duals from the manifolds out. Or use the inlets from the 94 y pipe and cut off the 6.2 inlets and weld on the 94s.

 

[Jimmy84]

I wish I could but I don't have the donor Y-pipe and at my age, I prefer the longevity of a single exhaust and a quite muffler.

 

[Grit dog]

An older set of small block manifolds should fit and have smaller diameter?
Seems you have to change one or the other.

 

[SirRobyn0]

Go to the auto parts store and purchase the flanges of the correct size, cut the old ones off and weld the new ones on. Or you could go to an exhaust shop and have them build you a set of adaptors, but it will push the pipes down a little bit.

 

[Jimmy84]

I looked at a set of exhaust manifolds pulled from a 1995 k1500. They were in good shape and I would have bought them except they were exactly what I have. Same bolt spacing, same pipe size.
The seller was adamant that these manifolds are standard issue (14093658) GM "factory bock huggers" and should work in my Jimmy no problem.
I wasn't the one who removed the diesel from my truck so I "assumed" it to be stock y-pipe.
It's probably not.
Ordered new y-pipe today. We'll see.

 

[Ricko1966]

Make new flanges. 2 piece flanges no welding required. 1 pieceof flat stock cut into 4 triangles drill bolt holes, hole saw center holes cut 1side out of each. Slip them over old pipes 2 per pipe ,cut sides opposing 1 another.Kinda tough to explain but not as difficult as you're thinking. If you start with the right size strap iron cut an angle turn it over cut an angle turn it over cut an angle turn the strap over so each new cut finishes your last triange.

 

[custodian]

I removed the diesel in my 85 C10 and it had dual exhaust. I replaced with a 350 and stock manifolds. The pipes I removed were close but didn't work.