Hi, I purchased a '99 YZ 400 that is in excellent shape (engine runs strong and body has not been abused). The gentleman I bought it from said he estimated it had 100 hours or less on it. My question, I am reading about complete rebuilds, replacement of valves and cams after 50 hours or less on these bikes. Is this if you are racing or really pushing the bike to its max? I do recreational riding and perhaps an enduro or HS once in a while. If I do the routine maintenance what sort of life should I expect from this bike? While I have no smoke or starting problems, am I wise to replace the piston, rings and knuckle now so I have a benchmark hours wise? Or just have fun and ride it until I notice start or smoke problems. I don't want to have it grenade and cost me a small fortune. This question is actually a good one for any second hand bike purchase, as I also bought a KTM 300 EXC that I have no historic records on. Your input is greatly appreciated.