I have a 05 yz450f dirtbike that has about 20 hours riding time on it, its not ridden hard and it blew the head cover gasket while riding at the track. The half circle rubber gasket on the top of the head blew out, I replaced it and made sure it was clean and torqued to specs but i didnt think of changing oil. I ran it for about an hour and it blew once again, the same one. After breaking it all apart and draining the oil there was some coolant in the oil. Does anyone have suggestions on what can cause the "head cover gasket to keep blowing out.