hi i have a 2004 400ex with a full 440kit wiseco 12.5:1 pistion ported and polished head, 185 jetted carb, procom rev box, and a full lrd pro4 exaust system, i run vp 110 fuel and i am haveing trouble keeping head gaskets in it i get about 3 good days a hard ridding out of a cometic gaskit i have a frend with a sparks 416 and he said to go to a white brothers gasket but i cant find them dose any one know where i can get a 89mm head gasket that will take the compression

 

You might need to change the head studs to the heavy duty ones. 11.5 and higher, is pretty tough on the stock parts.
http://www.gtthunder.com/TRX400EX.htm