exhaust manifold port size difference

[cadillac_al]

I am fixing an exhaust manifold on my K20 with a 350. I thought I would compare it to another one I have on hand from a 77 C10 with a 305. It is fairly easy to tell with the naked eye that the ports in my K20 manifold are larger than the 1977 manifold that I have. When I measure them they are about .050-.060" difference height and width. I was always under the impression that they were all the same size. The castings and markings are almost identical. Do you suppose some manifolds have smaller ports, like for a 305 or is it just manufacturing tolerance?

[MJN]

Thats interesting, can you post a picture comparing the two? I also figured they would be the same for SBC's. If you have the casting numbers I can look those up as well and see if theres anything different between the two.

[cadillac_al]

Everyone that looks at them says the difference is obvious.

[MJN]

I wasn't doubting there was a difference. If you had gotten the casting numbers off of them that will tell the original application of them. The manifold with the smaller ports has the support bars, those were added later on because the manifolds without them were prone to warping and breaking manifold bolts.

[cadillac_al]

I'm pretty sure the manifold with the bigger ports is the original to the 76 K20. This other one with the support bars came off a 77 C10 with a 305 but I have no idea if it is original or not. The numbers on it are L1177 346747LH. I just can't figure out if the size difference in intentional or production tolerance.