cracking exhaust manifolds

[Ken Keith]

Crap.  So these manifolds crack.  I thought the ticking was due to 
warping, which could be fixed by milling the surface and putting it 
back on.

So if mine's potentially cracked, where is it visible?  From the 
bottom?  Is it something I can keep from getting worse?  Is it 
caused by too-short cool down times?

Is this cracking something that, once started, the manifold is junk?

Thanks for all the responses.

Ken