i have an 01 wr426f and i bought a new timming chain for it but havent installed it yet. im planning on going riddind this weekend and im scared i could possibly blow up my engine if i dont replace it. i bought an 02 yz426f in the summer and the timming chain skipped and it broke a valve and ruined the head. i heard every 40 or 50 hrs oh ride time. the 02 yz426f i had was bought new in 05 so it was only ridden for 4 years. how often should i replace it? how long does it take to replace?