I just finished my top-end rebuild on my 01 XR 400. I bought it earlier this year. It started & ran very well, but it did seem to use more oil than Id cared for. Aprox 1 quart every 1000 miles. I decided to go with an Polydyn coated Wiesco 426 kit. 11:1 compression. I also replaced all valves with new Kibblewhites. The valve guides mic'd out like new so I replaced the valve seals only. I've been breaking the bike in for the last few days. The increase in power is very noticeable. Very big increase in torque right off the bottom. Wheelies were amazingly easy in any of the first 3 gears. Top end there was really not much difference. Im sure a cam would open up the powerband well, but Im pretty happy with just the piston thus far. The bike starts just as, if not easier than before. I am running it on premium 93 octane fuel & haven't noticed any problems with detonation. Its definitely well worth the money for anyone looking for a relatively inexpensive mod to do. Especially if you are due for a top end.