I noticed my 2006 WR450 was leaking a slight bit from the head gasket area. I took it to the mecahnic who put a big bore kit in it for me last spring and when he tore it down he said the head was cracked! My question is, what would cause the head to crack? The obvious answer is overheating, but, even though I did ride several times on slow single track in hot weather this summer, it never steamed or boiled over. I did have to top off coolant here or there, but nothing out of the ordinary. I'm concerned about the money of replacing the head, of course, but I'm just as concerned about how to avoid this major expense in the future.