Hmmm ..New theory(?): Could it be the 'shields' causing warped manifolds??

[Grams]

Annealing & stress relieving are different processes
Annealing will alter the material making it less hard.

I new that.

(I also knew that)

I misspoke. thx

(However,…purely for the sake of discussion…. the Ordinary/Everyday operation of the engine is quite-naturally …and more-nearly …the “stress-releiving” process in-itself….as the manifold cool-down is protracted. *
While taking the manifold OFF the vehicle and heating it to higher temperatures may/would alter its crystalline-structure…..but require keeping the higher temp longer….. and do nothing for it’s size and shape.)

* the problem may actually be that because it is bolted…and before the bolts break…the manifold cannot fully relax to it’s stress-relieved shape…. and/until repeated cycles break those bolts. That is why a manifold that is re-used after the bolts are replaced may often perform just fine. LOL

 

[Hagar1]

I'm not sure how cast iron is processed, but in our machine shop at work, when we make cylindrical arbors from aluminum, we always machine them slightly oversize and then bake them for several hours to stress-relieve them before they get machined to final size.

If they are machined without heat cycling before final sizing, they will not hold concentricity aftet the first use and will be NFG.

When we flatten copper wire, it gets work-hardened and has to be baked for several hours to anneal it for use.

The way that the exhaust manifolds warp like hell, then stay good once milled/sanded after being heat-cycled makes me think that it was never done, or never properly done at manufacture.

You are 100% correct! The process is cast them, machine them and throw them onto a vehicle or onto the sheld.
Not manifold related but back years ago Mopar performance (Direct Connection) offered engine blocks that were stressed relieved at the factory. Could go that way or just get an engine block with a hundred thousand miles on it, because ,t too would be stress relieved after all those thermal cycles.

 

[04fxdwgi]

You are 100% correct! The process is cast them, machine them and throw them onto a vehicle or onto the sheld.
Not manifold related but back years ago Mopar performance (Direct Connection) offered engine blocks that were stressed relieved at the factory. Could go that way or just get an engine block with a hundred thousand miles on it, because ,t too would be stress relieved after all those thermal cycles.

Yup, Grumpy Jenkins made "seasoned blocks" famous, Before I rebuilt engine blocks, I used ones that were outside in the weather for a long time.

When in the Navy, stationed at Groton Sub Base, made a torque plate for machining my 60 over Hemi block out of a 3" piece of HY80 sub hull steel. Bolted on, with gasket, and ran 200 deg water through it during the machining cycles. Block was perfect when assembled and at temp.