This by far has been the biggest headache for most 400EX owners. I have read so many posts on here about blown head gaskets, I would like to give a few tips on what I have seen and what I have done here at Four Stroke Tech to eliminate head gaskets problems after boring or re-sleeving the 400EX.
After you remove your cylinder, the first thing to do is to find out if your cylinder is any good and if the studs have either come loose or are pulling out of the cylinder. Before you do anything, torque the studs by putting 2 nuts together (stock 400EX nuts off your wheels) back to back work good. The cylinder studs should be torqued anywhere from 35 to 40 foot pounds. If they won't hold this, your studs are pulling the threads out of the cylinder. If they don't hold, you can either go from a 10mm stud (which is stock) up to a 12mm after-market stud and re-tap your cylinder. I have had great success with using Time-Certs which is a heavy-duty helicoil. If your cylinder holds the torque then go ahead and remove them and if you are just boring your cylinder, clean the studs and the threads in the cylinder with acetone (or some kind of de-greaser). I use Honda high-temp lock tight liberally on both the stud threads and the threads in the cylinder re-installing them and torquing them to spec. After this, go ahead and bore your cylinder to spec.
