Pipe bender

I've heard the same thing, but I think it's just an old tale that got started on the dead pecker bench in front of a store when some kid went by with open headers....lol. Supposedly, the cold air will run back up thru the headers/manifolds and when it hits a hot exhaust valve....it will cause it to bend. It never made sense to me, because when you shut a motor with headers off.....the headers will still burn the piss out of you 20 minutes later! If they stay that hot, for that long, I would think that any cool air that went into the collectors is gonna get heated up long before it finds an exhaust valve. That's just my hillbilly way of reasoning, and I may very well be wrong...lol.