Hey guys I recently picked up an older (98/99) yz400f. The last dirtbike I had was more than 10 years ago and it was an old bike then...an 87 cr125, so just about anything feels new to me. The bike is very quick by my standards and has all the cool stuff I drooled over years ago, so I'll be happy for awhile with it I'm sure. I have been doing some research on the bike, and the early evolution of the 4 stroke yzf lineup, and it seems like a pretty impressive piece of machinery. A little background for myself...my background is in cars, and I am a mechanical engineer, so with this combination of traits I am naturally inclined to mess with my toys. I'm certainly no stranger to the 4-stroke engine, in fact they are much more familiar to me than 2 strokes. In my initial research on this bike I have found that in 2001 the yz426 switched over to titanium valves. Now the bike that I have is plenty strong, and runs and starts quite well (I do think a good carb cleaning/rebuild is in order though). I have noticed that after sitting at idle for more than a couple of minutes that it starts to puke coolant. I also have read that this is a common issue, so I will go through all of the normal checks to make sure everything is up to snuff. I was thinking that in the worst case that the head gasket could be bad or the head could be cracked. So this brings me to my first question about this bike. In the event that I had to pull the head, it seems like a worthwhile upgrade could be switching to that 426 head with the titanium valves. Do you guys know if the 426 head will bolt on to the 400 cylinder?