<VV> Warped Carbs?

[grant young]

Looking for educated opinions on what makes the Corvair carburetors warp 
in the throttle body area? I just tossed out several secondary carb bowls
in which 
I could not get a good throttle plate seal because the sealing area was so
far 
out of round. I find them from perfect to about .020" or so out of round. 
It seems like their mounting design and the throttle plate would prevent
this. 
While the warp is usually not enough to make much difference in a primary
carb, 
it can be death to a secondary. I know that aluminum gets happy around 350
degrees, 
so I suspect it is a design oversight?
Thanks.
Grant