I finally got around to tearing my motor down on my 400 ('99) this weekend. It blew on me about a month ago..... as I suspected I dropped a valve on the piston. http://i14.photobucket.com/albums/a306/rockieman01/wr400%20motor/IMG_2080.jpg I was able to dislodge the valve which disclosed a hole in the top of the piston. Looks like a new head is in order as well....the seat for the valve that broke got hammered pretty good. http://i14.photobucket.com/albums/a306/rockieman01/wr400%20motor/IMG_2077.jpg In order to keep my cost down I'm thinking I'll stay with the standard bore and just replace the piston. I'm looking for suggestions on what I should do with the head? New? Ebay? I also would appreciate input on whether or not I should split the cases? When I drained the oil it was like new....only had 30 miles on the last change. Thanks in advance for your input!