I have an 06 YZ450 that I ran for about 40 miles with no coolant. I was on a ride recently and everything was going great. I was running the bike hard and coming around a corner and the rear started to slide out quick. I saved it barely and then the bike started to misfire badly. I first thought I must have broke something when I got sideways, but I did not over rev or anything. The bike just started to cut out and then it finally stalled. It was very hard to start, but it ran. It was fine down low, but once I got the RPM's up it was just popping and cutting out badly. I did manage to limp back about 20 miles just cruising in 2nd. I had to crank up the idle to prevent stalling, but it made it back. Since the bike had 80-100 hours I was getting ready to refresh the top end (just for preventive measures). I warmed up the bike so I could drain the oil and the bike was running fine. I could run it hard without any of the issues I had on the last ride. Once I started the tear down I found out the coolant was completely dry! I figured I was going to see some piston or valve damage. I am running Amsoil Synthetic in it. I could not find any wear or damage at all! I was shocked that I could run it that hard without coolant and not cause any damage:excuseme: . My only guess to the cutting out and popping is that the head was getting so hot I was detonating the fuel from the head being so hot. Not sure if thats possible, but once I refreshed the top end it ran like a champ once again. I did find the O-ring was cracked on one of the inlet pipes and that caused the coolant loss, so at least that has an explanation. Anyone know or have this happen to them? I am swearing by the Amsoil after this though. I think that may have saved me from having even more damage!