My son who is in the Navy wants to know how often he needs to check valve clearences, do a valve job and/or do a top end rebuild. He would also like to know what the symptoms are of a bike needing a valve job. The bike is a 2004 yz450f. Since he's not riding much right now he thought this would be a good time to make sure its in tip/top shape. Any feeback would be very much appreciated.