Definitely have to pull at least one head on mine (as far as I can see) - the valley tray dovetails under the heads on both sides so can't be lifted, even if you can break the RTV. Possible you could slide it but I'd have to pull the rad and everything for clearance. Oddly enough, I was planning on fitting the one with the inspection panel but I didn't anticipate how regularly I was going to have to inspect....