On some of the valley covers they will only go on with the heads off. Test fit the cover with heads on and trim the sharp edge with a wood joiner, file, etc., until you can slip one side under the head and then drop the other down. Slide it to the center and fill the gaps with silicone (as well as the ends). On one cover I had to just trim the knife edge and on another I had to cut the bevel edge as well.