I have a 2000 400ex that I put a 440 big bore in. I put the big bore probably 6 months after buying it brand new. I've never had to tear it back down to fix anything. I don't race, I'm just a weekend warrior. Mainly do dune riding, trail riding and the occasional track. I have air shrouds, added an extra oil cooler, added a larger oil tank and only run 110 octane. I use Honda oil and change it after every 5 rides. Its going on 13 years and it still starts and runs good, I finally had a shop do a leak down test and they said everything looked good. Anything you can do to keep it from getting to hot is good. I think that's why mine lasted so long.