I'll pass along what we used to do at the mill when working with super hard brittle material. Try peening the weld with the pointed end of a chipping hammer. Just keep hitting it until you figure the entire weld has been gone over. Then do your slow cooling. It worked for us. Good luck

I am giving up on this :( Pre-heat, small welds, peening, slow cool. All did not work. So disappointed. Anyone have a non-cracked 3942527 manifold they are not using? :D

FWIW the exhaust manifold with the "raised walls and rivet bosses" for the preheat shroud and without the provisions for smog are not correct for any 69-73 Z28.

I had a similar crack on a small block manifold but was on the portion that bolts up to the header pipe. I never repaired and had no issue with it all.
I have about quit messing around with 50 plus year old exhaust manifolds and have been purchasing the repros. The casting numbers in most cases are correct, and the date code can't be read when the manifold is on the car and its nice to have a part that has not heat cycled dozens of times. Just my .02 worth.

I was just using this "other" cracked manifold as a test candidate. I have a SS350 4-speed, so it uses the same smog manifolds as the Z.

Buying a repro might be the way to go now and just use the one original side that is still good.

Thanks - I kept thinking to myself "I swear the crack was not near that bad".

The only alternative I can suggest is brazing the crack. Braze, while not as strong as weld, is pretty forgiving for cracks on cast iron since when the cast iron cools it will "stretch" to the surrounding shrinking iron rather than pulling the iron. You can ceramic coat it afterward to cast iron color (I've had good luck with Cerakoat Glacier). I've seen punch press frames brazed together that have lasted for years afterward. The manifold would be much less stress than that. Typical heat on the manifold is less than the melt point of braze. Maybe worth a try.

Thanks Dave. I think I have some brazing rods left over from another project to test that. What can it hurt? :)

Kevin, there is a guy up near me that knows how to weld cast iron. His name is Steve Babinsky from Automotive Restorations ([email protected]) (908) 236-6400. His cars go out to Pebble Beach. You might want to talk to him about the cast iron weld. He told me the cast iron welding rods you buy are from CHINA and are junk. He has his welding rods made here in the USA.

OK, I said I was giving up, but I just can't let a smog manifold go unrepaired :) So was doing some other tests and welding different cast iron stuff.

The "special" nickel wire was just not sticking to the parent metal. So in the "what the heck" moment, I just used the regular .025 mild steel wire. It seems to stick much better and don't see a join crack (yet).

So I went ahead and did the smaller of the cracks on the smog manifold. Its heat and clean 1st. Then when warm to touch, small beads, and grinding, then another bead and grinding, etc, waiting for it to be warm to touch each time.

Seems to stick properly. Finished it up with some die grinding and media blast. I will let this sit over night and then heat it up with a torch to see if any cracks appear.

Not sure why I am getting the voids, I ground them out between welds. I found after the first beads that if I start the weld on a grinded bead it flowed to the cast iron better than starting on the iron. I will work on the mating surface more if and when I do the 2nd crack. That will get a gasket anyways.

Opinions welcome as always :)