63-65 3797902 exhaust manifold counterbore

[Joe R.]

I gather that the 2.5 inch 3797902 exhaust manifold casting was used on both the left hand and right hand sides of various 63-65 small block applications.

Normally, a left hand manifold would have the flange counterbored for the "donut" exhaust seal, while a right hand manifold would have a flat flange to accept the flat gasket associated with the heat riser valve.

Was the 3797902 manifold machined two different ways, depending on whether it was intended for lefthand or righthand use?

I know that a heat riser can still be bolted up to a counterbored flange, but I'm not sure that the gasket would seal well.

[Mike M.]

Re: 63-65 3797902 exhaust manifold counterbore

flat gasket between heat riser and exh manifold will seal fine. mike

[Mike M.]

Re: 63-65 3797902 exhaust manifold counterbore

flat gasket between heat riser and exh manifold will seal fine. mike

[Joe L.]

Re: 63-65 3797902 exhaust manifold counterbore

Joe-----

I don't know of any original left side applications for the GM #3797902 manifold. The 3797902 can be mounted on the left side, but I don't know that the factory ever did that.

Nevertheless, I believe that the 3797902 does have counterbored outlet to serve as the seat for a "donut" exhaust packing. This does create a very slight problem when a GM #3981068 (no longer available) gasket is used since about 1/8" of the inner circumference of the gasket is consequently and directly exposed to exhaust gas heat. The GM gasket does have an inner steel shield but that will burn away rather quickly in the exposed condition. Regardless, though, the remainder of the gasket will seal the joint between the manifold and the heat riser valve (or, spacer) with no problem, at all.

[Joe L.]

Re: 63-65 3797902 exhaust manifold counterbore

Joe-----

I don't know of any original left side applications for the GM #3797902 manifold. The 3797902 can be mounted on the left side, but I don't know that the factory ever did that.

Nevertheless, I believe that the 3797902 does have counterbored outlet to serve as the seat for a "donut" exhaust packing. This does create a very slight problem when a GM #3981068 (no longer available) gasket is used since about 1/8" of the inner circumference of the gasket is consequently and directly exposed to exhaust gas heat. The GM gasket does have an inner steel shield but that will burn away rather quickly in the exposed condition. Regardless, though, the remainder of the gasket will seal the joint between the manifold and the heat riser valve (or, spacer) with no problem, at all.

[Joe R.]

Re: 63-65 3797902 exhaust manifold counterbore

Hi Joe:

Sorry, I meant to say 3797942. This is a symmetric 2.5 inch outlet manifold with no alternator mount, so it could be mounted on either side. I believe it was used on the left side of fuel injection engines, and the right side of engines equipped with A/C. The right side application was probably the dominant one in terms of production volume.

The Paragon reproduction of the 3797942 is not counterbored for the donut seal, making it a right-side-only manifold. The used 3797942 manifolds that I have seen on ebay are counterbored for the donut seal. This is what led to my question.

I realize that it is possible to use the counterbored version with a heat riser and get away with it, but as you point out the steel shield on the gasket will be exposed. I would prefer the flat flange version for a right hand application.

I'm curious whether GM actually used the counterbored flange on right side applications, or whether the original right side application for the 3797942 casting had a flat flange that was designed to mate with a heat riser.

[Joe R.]

Re: 63-65 3797902 exhaust manifold counterbore

Hi Joe:

Sorry, I meant to say 3797942. This is a symmetric 2.5 inch outlet manifold with no alternator mount, so it could be mounted on either side. I believe it was used on the left side of fuel injection engines, and the right side of engines equipped with A/C. The right side application was probably the dominant one in terms of production volume.

The Paragon reproduction of the 3797942 is not counterbored for the donut seal, making it a right-side-only manifold. The used 3797942 manifolds that I have seen on ebay are counterbored for the donut seal. This is what led to my question.

I realize that it is possible to use the counterbored version with a heat riser and get away with it, but as you point out the steel shield on the gasket will be exposed. I would prefer the flat flange version for a right hand application.

I'm curious whether GM actually used the counterbored flange on right side applications, or whether the original right side application for the 3797942 casting had a flat flange that was designed to mate with a heat riser.