Anyone ever have an issue with a threaded valve cover. One of my machines (2000 Magnum 325) blew the exhaust valve cover off the other day. There was some RTV sealant on it and the o-ring was still on the head. I cleaned it up and tried to reinstall it but it doesn't seem to want to tighten up real tight. I put it on as snugly as I could but it blew back out upon engine start. It doesn't seem damaged and neither do the threads in the head. Is this sort of thing common? What's up??