[Lee H]

Originally Posted by mechanicalman

That appears to be a R hand manifold, unless the photo is reversed. On my 72 the L manifold did that, but I assumed it was because of the "hot air" tube sucking cold air across the hot manifold, and it was not equipped with a heat riser (EFE valve) from the factory but it did have an emissions device that locked out the vacuum advance when the engine was cold or until it hit 3rd gear so that I think might overheat the exhaust gasses.

So, on the R manifold, did you notice if the vehicles that had the cracked R manifold have a heat riser valve?

Just curious why this happens. I've see AIR cause this on big blocks.

Never paid that much attention but have always speculated it was from heavy towing/hauling heating up the manifold. I remember our tractors manifolds would be cherry red during spring plowing, and fall turning. Just assumed it was about the same thing with heavy towing and hauling with a truck.