Trap Door Inside Exaust Manifold

Modesto is in Californias Central Valley. It rarely gets below freezing, supposed to be 92 tomorrow.

I just finnished curing the VHT paint on the manifolds with a heat and cool cycle in the oven. When I pulled it out of the oven at 500 degrees the door was about 75% open. How hot do manifolds get? I would think 500 degrees would be hot enough to open the door compleately.