Thechinspector hit the nail on the head. If you allow cold/ambiant air temp to get to a freshly scalded valve the fast cooling will crack/warp the valve. If you run at least a couple feet of header your all fine, I ran a pulling truck with a small rust hole in the primary just outside the flange and it went all summer. I would never start an engine I liked and run it hard with no manifolds/headers and shut it off, your asking for trouble. BTW, you would also be throwing flame out the ports a few inches if you ran open heads.