here is a picture of the flaps:
http://www.highperformancepontiac.com/tech/hppp_0802_pontiac_cooling_system_upgrade/photo_09.html
this is an article on electric fans shrouds I found while looking for flap pictures:
http://www.highperformancepontiac.c...ng_system_upgrade/aluminum_radiator.html

At the bottom of this page is another picture of the flaps, otherwise it shows a bunch of ways to mount a fan:
http://www.spalusa.com/fans/mount_options.html

The flaps are not spring loaded, and they don't really work by gravity. I would say that they are just plain check valves for air, and I suppose gravity keeps them closed at rest, but the air pressure is what really operates them when in use.

Now after all that, I still don't think you need or even want a shroud on the front of a radiator, I can't see how it would help, with or without flaps.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-