I have just recently bought a 2003 400ex. This bike has alot of mods like yoshi pipe,160 main jet, outerwears airbox lid, works shocks, renthal handle bars, skid plates, a weisco piston, sprockets,a arms. The thing is i have know idea what compression im running or how big the piston is or how much it has been bored out. I have a leak around my head gasket although it is not bad it is still there and is is blowing little oil out from time to time but not much. So if i get a repair manual and a new head gasket would this be a hard task to do? Do i need to order a special head gasket? How do i tell how much it is bored out? The thing runs really good as far as i know really strong. . Thanks

 

I have decent machanic skills but i have never worked on a quad engine before. Tips and help would be very helpful.